I'm just going to focus on this part, because you keep missing the point. I haven't disagreed with either 3 or 4. What I've pointed out is that IN ADDITION TO THAT, blocking terrain or an enemy on the other side of grease can't be walked into. So if the grease can't be jumped across, it ALSO can't be walked across, so there's no reason for the 'giants' to try to walk past it. Saying 'ah-ha, you can't jump over this obstacle because there's solid walls on the other side, you'll have to walk through it' begs the question of why you'd want to walk through it if there's solid walls on the other side.
To start. Here is my contention after GMing a Swashbuckler + Champion + Diviner for probably 150 hours intermittently from levels 4-7 then a ton from 11-20 (various intervals...again, I was filling in for a GM to make sure a group of teenagers' game got played) back in 2016-2017.
Grease is fine at low level.
Curiously, the problem comes in at high level because (a) of spell proliferation making the opportunity cost of loading it out miniscule (and therefore having a huge answer for a specific encounter situation - chokepoint management + kiting enabler via amplifying a terrain configuration vs low Dex Brutes) + (b) the spell DC is so high + (c) having answers to all encounter types in your enormous suite of answers (+Signature) + your enormous gastank (massive # of spells, huge gastank with spell slots + Recovery + Mastery + Rituals) is what makes high level spellcasters overpowered. Portent is just the icing on the top for Diviners.
Now, onto what is being missed here:
* Your optimal path of movement to a high mobility + kite-capable target is (say) 6 squares (within your move).
* A spell that plops down a 4 SQ control zone which connects other pieces of continuous terrain punishment then turns your 6 squares of movement into 8 or 9 (as you have to reroute from the prior optimal path)...now you're screwing with action economy sufficient to equal action denial of melee multiattack by proxy. Your left with a catch 22 of spending action economy to Dash and hope you can close to melee to deploy multiattack...except the Swashbackler can just dance away without OA and the Wizard has Misty Step at-will (so both have AoE avoiding Dash at-will). So you're in a position of pretty much being locked out of multiattack...so your potential damage output is wrecked.
* This terrain-abuse (lets call it) makes you hugely kiteable (which crushes your potential dps output).
Finally, no you can't just horizontal jump over anything that is prone. The alien ATST proned in the hall because of a readied Grease is the obstacle. How are a bunch of ATSTs leaping over a prone ATST in a 10*10*15 corridor? That is some beyond Matrix level wire-fu there. The PHB says you can attempt to clear a "low obstacle" and puts the clearance max for the low obstacle to a horizontal long jump at 1/4 the jump; otherwise you hit it. There is no way a Large or Huge creature proned in a hallway on Grease is a "low obstacle". This basically means that somewhere around 5-7 ft is pretty much going to be the absolute max a reasonable creature can clear in a horizontal long jumps (basically the world record longjump in our world...which is what DC exactly in 5e?...when it doesn't have a ceiling for a 12 ft ATST to contend with?) and you're putting a huge DC on the Athletics check (or they hit it) or you're outright ruling you can't jump it (because its not a "low obstacle"). These tanks would weigh an absolute F-ton and their girth would be huge even if prone. Clearing that looks to be outside of what should be reasonable or at least a significant Athletics DC if you can even try it. Couple with the corridor for the ceiling? If you're a GM who is allowing that outright or putting a low/medium/or even high DC on that I_never_ever_ever want to hear about punishing DCs for Fighters or Rogues when they're trying to do epic level athletic/acrobatics maneuvers (their power : weight ratio compared to these tanks is PROFOUNDLY in the favor of the Epic Fighter/Rogue....yet, these ATSTs are supposed to just pull off a simple leap move here?).
If they can't jump it and you're ruling that they can indeed treat it as "difficult terrain", then they're saving against Grease (which, personally, I think that is a hugely contentious ruling...out in the open, not in a corridor, not a creature who takes up nearly all the space of a corridor, w/o a terrain hazard like Grease...sure...when everything is pointing the opposite direction...questionable ruling in my opinion). Either way, its a train-wreck of action denial by the Tanks. That is how Grease can create a huge chokepoint and debilitate the action economy of reinforcing large creatures w/ bad dex in a dungeon corridor.
And linking terrain features by way of a strategically placed 4 SQ zone which complicates your path to your potential melee multiattack targets by even +15 ft (or 3 SQ) can create action denial and turn melee multiattackers into single target ranged attackers as they're forced to deal with terrain + movement enabled kiting.
+++++++++++++++++
Again, the problem isn't low level Wizards with Grease. Its high level Wizards with Grease (because it gives them a hugely effective, nil-cost, tool in the toolbox to seriously damage encounter archetype (which is exactly what you're looking for as a high level Wizard...Rock for when Scissors is thrown at you, Paper for when Rock is thrown at you, Scissors for when Paper is thrown at you, the ability to surveil so you know enemy weaknesses/what problems you're going to face, the ability to dictate Long Rest Recharge, and tactical Nukes when you feel like it).