I've never understood that argument. It was around during the 4e edition-wars, and I see that it is alive today.
I still don't get it.
The 2014 stat-blocks are a bunch of words and numbers that supposedly represent a creature. The 2024 ones are also a bunch of words and numbers that represent a creature.
What's the difference?
How does that difference make one a "creature" and the other a "collection of abilities"?
In traditional D&D, Humanoid monsters started out with a base set of stats. Then you'd tack on PC class levels (!) to make them more powerful.
This inherently made the Humanoid monster's upgrade feel like it was part of the world - it was casting spells that the PCs cast and recognize, it was doing moves that PCs did (or could do), etc. A goblin rogue backstabbed like a PC did, a goblin fighter got extra attacks like a PC fighters.
Now, a problem develops in that the "ideal" PC is full of bells and whistles for a single human being to spend time tweaking. The player also tweaks the PC between fights and sessions, and PCs last for a long time. Monsters, on the other hand, are "ideally" often run by the half dozen or more by a DM, and rarely survive an encounter.
This means that the detail level of a PC should be
higher, to keep the player entertained, while the detail level of a foe of a PC should be lower.
This became exceedingly clear in 3e, especially with spellcasting foes. Building a single spellcasting foe could take hours or days of a DM's time, and running them was also complex.
In 4e, they reacted to this. The monsters where built as an emulation layer; the idea is that you build what the player experiences and you leave the other details up to the DM. This means your monster abilities and PC abilities no longer pull from similar pools, and indeed could use utterly different mechancis!
If you do this well, there is no way to tell this is happening; but it is very easy to "mess up" and make the lack of detail in the emulation leak through. When this happens, the monster can feel "gamey" instead of like a creature in a fantasy world.
Some of the early attempts to bring this over to 5e had the same problem. They had spellcasting creatures who had this "arcane blast" ability that didn't match any ability a PC had. It was a generic combination of a cantrip and a low level spell, with the idea that by the time you fought a mid-level spellcasting foe, them spamming 1st or 2nd level spells every turn in combat is reasonable. The damage type was "DM just pick".
Everything you need, as a DM, to emulate a necromatic acolyte or a druid or a fire mage. Just pick an appropriate damage type and tier of foe! Then maybe tack on some extra abilities.
But "arcane blast"
wasn't marked as a spell, so counterspell didn't work on it, in at least some cases. Oops! A leak in the abstraction. And DMs wheren't told "make it a necrotic blast, consistently, for cultists, and rename if needed", so would talk about this strange cantrip that sort of kicked ass that PCs couldn't duplicate.
The emulation leaked.
More than that, the monsters wheren't well designed. Lighting is supposed to do more than just damage - for it to feel like lightning damage, it should shock, stun, penetrate metal armor, etc. Fire damage should set things on fire; cold should slow people down. Poison damage should risk being poisoned, necrotic damage should sap your will to live, acid damage should corrode over time, etc etc.
You can bring all this
back to the generic "CR 4 spellcaster", but the laziness of "do we haaaavvee tooooo?" kicks in. And you get sub-par cardboard cutout spellcasting monsters who don't feel grounded that cast Arcane Blast with acid damage that is mechanically identical to the cold damage you got last fight.
Being forced to actually pick PC spells (and abilities) means that the work put into making PC spells "feel like that damage type" gets delivered into your monsters (that is, when PCs spells are well written). Being forced to us PC classes for abilities similarly makes their combat abilities feel coherent.
You can see what happens when 5e does a good job of this. Look at the 2014 Guard, Veteran and Champion monsters; these are great emulations of Tier 1, Tier 2 and Tier 3 fighter-types PCs. When you fight them, you feel like you are fighting somethign of similar competence; maybe they lack some training, or have some subclass you don't have, but they don't feel utterly alien.
When the 4e style shines it creates great emulations that are fun to play against and with the DM. They don't have gaps that make the emulation show through on the PC side, and give good advice for customization to the DM.
I want there to be 3 or 4 common kinds of goblins, like this:
Goblin Stinkers - low end, not very competent, melee grunts.
Goblin Sneaks - archers, sneaky, etc. Rogue inspired.
Goblin Beast Riders - ride wolves and dire rats, cavalry. Ranger inspired.
no heavy infantry, because goblins.
Then some leader types:
Goblin Shaman - Druid/Cleric inspired spellcaster
Goblin Alchemist - Artificer/Sorcerer inspired spellcaster
Goblin Chief - Tough, Sneak/Beast Rider combo.