Some had an issue with Yuji labelled as a martial artist?

Ch1 rumors of Yuji's athleticism are going around the school, this would imply that he has recently done things to ignite those rumors. Something he couldn't have done if his grandad was in hospital for a long time,

as he goes to see him daily at 5pm.
Also during ch1, there is no mention of financial trouble when his grandad died. I assume his grandad was hospitalised recently & they have a stable place to live. & stream of income. Maybe a dojo etc where

Yuji could have thought beginner classes to younger students as inheriting such practices is common traditionally.

Ch11 Yuji is skilled enough to dodge a move from a heavily suppressed Sukuna & aim a manji kick in retaliation.

Ch16 in order to take Yujis existing abilities & bring them up to usable levels, Gojo decides to give Yuji an intensive 1 month sparring program where he definitely constantly upped the intensity. Considering he thinks Yuji will be one of the ones to match him in the future.

Ch33 Megumi remarks that Yuji could take all people from Kyoto & Tokyo joining the goodwill event, if they were using straight hands only.
He says this as his assessment before Gojos intensive training.

Ch50 Yuji matches the Black Flash record. Momentarily after achieving it for the first time. The level of awareness he has in his own body and its capabilities potentially rivals characters like Toji if we are to take the Black Flash lore as stated.

Ch61 Yuji achieves Black Flash almost willingly, having the resolve to end the battle before Nobara needed him to. The sun which is connected to the manji sign is again shown here, matching the moment of clarity Yuji had against Sukuna.

Ch103 Yuji jumps in the air in order to build momentum & dodge the blood, a technique later revealed to be a staple in his martial art. Choso only mistakenly assumes Yuji did this in an effert to goad him.

In general, Yuji takes a fighters stance whenever he's in danger, in comparison someone like Nobara only takes a alert, battle ready pose. Yujis stances are constantly repeated, hinting at a trained response. Although not explicitly stated, the signs are abundantly clear.

I only covered my favourite moments from memory