I believe the intent here is to suggest that the encounter includes 4d6 quantity of goblins.
So 14 goblins, which, at most, the fighter could attack around half of them on a turn assuming they are conveniently spaced.
At 7 hp a pop (CR 1/4 goblin), max modifier+basically any static damage bonus means any hit would indeed kill the goblins.
It does mean that the fighter is wasting a ton of damage on overkilling some goblins that could be going into the tarrasque (This is neither here nor there, as the same would be true of minion).
But here's the thing. That 7 hp goblin was pretty one-shottable at level 1. A +3 modifier + dueling fighting style means it only survived with a 1 on the damage roll 19 levels ago.
So let's look at the advancement from level 1 to level 20. At level 1, just starting out, a fighter can likely kill 1 goblins (assuming hits a hit). At level 20, at the pinnacle of their adventuring career, with an action surge, they can certainly kill (on hit) an incremental 7 more more goblins at level 20 on a turn that they could not kill at level 1.
That works out to..1 additional CR 1/4 creature a fighter can take out a turn (with action surge)..every 2.5 levels.
Is the heroic journey people should expect for their fighters, from being able to kill 1 goblin when they're first starting out to being able to kill 8 goblins at peak 'heroes of the realm' physical prowess?
Goblins are probably a bad example for this exercise as they actually are one-shottable. What is the highest CR creature a fighter can reliably one shot at level 20 that they couldn't one-shot at level 1.