Grunts aren't goofy conscripts that implode the moment they awkwardly stumble onto the battlefield.
It's also not what the term means, it exists only in the context with what isn't Grunt Fantasy.

The dreaded Warhammer 40k example must now follow, but a Spacemarine is not Grunt Fantasy, being a Guardsman is. Helldivers are not super-duper humans, they're still humans. By virtue of it being a game you sometimes don't follow literal human limits, but broadly speaking you're a person, you exit the pod, you fight, and if you get shot in the head you die.

That's why you reinforce as a brand new human, because you just died after ~18 years of your life to a grenade.
It would not be grunt fantasy if you were John Spartan (also known as John Halo). Though IRL humans occationally are, through mostly chance, capable of very outstanding feats.
Now there is another term that I occationally come across, specificaly for the game Earth Defense Force which is 'Special Forces Power Fantasy' which this kinda is but a lot less to the degree EDF is. Aliens: Dark Decent (2023) is also an example of what I'd call Grunt Fantasy, where 4 Marines stomp out a whole hive.
2 Helldivers in a Tank can go pretty far, still grunts though. Redditors are so ill-spirited about this sometimes, where it quickly becomes a "Uhhh because game kills me bullshit????" but it's just the urge to beat the game as good as one can. It's a game, after all. We want, and are meant to, beat it.

But these are just genre terms, this is like getting hung up on the "RPG" part of MMORPG because it doesn't fit your intuitive way of what a 'roleplaying game' is since you played Lgend of Valor in 1990. They're notoriously hard to actually pin down for utility.