* 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.
You just jump over the grease effect if it's in your optimal path. If someone does the readied spell thing to catch the first 'giant', then that giant slips and falls blocking the hall... then uses it's action to dash, which lets it stand up and move 3 squares out of the way and clears the hallway for its buddies. So you've used an 18th level spellcaster's action to block one enemy from getting attacks for a turn. That might be a reasonable trade, but it isn't 'wrecking' anything about the action economy, you're trading one PCs main action and reaction (and, again, in a lot of level 18 fights giving up the ability to counterspell for a round is a big deal) to stop one enemy's main action.
If you're they can't jump over it because you've decided the roof is too short, I'd say that the scenario is too contrived to be a general rule. I'm also not really sure how the 4 giants with a 30' move the lead one of which needed at least 20' of movement to get to a PC who is in an area that's generally 10' wide were going to gang up on a PC without grease blocking one. It sounds like the best they could do without grease is get one giant engaged with a PC and the rest stuck behind him, or maybe have one squeeze next to the PC and a second one engage the PC with the other two behind. 30' move large creatures can't gang up very well in 10' wide corridors.
* This terrain-abuse (lets call it) makes you hugely kiteable (which crushes your potential dps output).
Move 30 creatures against level 18 PCs are generally highly kiteable unless there's something about the terrain that forces the PCs to fight instead of kite. The grease spell doesn't have anything to do with that.
Finally, no you can't just horizontal jump over anything that is prone.
You're wrong about this, but I deliberately cut those points out of my response because I'm not interested in arguing about the jumping rules, and literally none of what I'm discussing involves jumping over a prone creature. There's no reason for one of the giants to lay prone in the grease, they can always dash, stand up, and clear the way for those behind.
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, it doesn't complicate their path. If one gets caught by a readied action casting then he dashes to clear the way (effectively trading one PC's action and reaction to stop one enemy's attack sequence), and the others just jump over the grease. I don't think that using the dash action and 'no roll required' jumps is anything special, and 18th level characters kiting 30' move enemies who are in a space that is more confined for them than for PCs is generally going to be pretty easy.