This particular sentence was just horribly written due to redon's complete nonsense agenda of trying to call Yamato "he/him" in every single instance possible.
It's not even about calling Yamato "he/him" in general.
I personally think that's ridiculous, since Yamato has been confirmed as a bonafide girl in the vivre card (only addressed with female pronouns there) and in the official promo. Not to mention that she is addressed with gender neutral pronouns in the japanese version of the manga, so the statement that "everyone uses male pronouns" is wrong anyway.
However, if someone still wants to use "he/him", ok, whatever. I cringe and don't comment on it.
With redon it's the frequency and the obvious attempt to force-feed his readers this gender-non-topic that annoys me.
In this particular instance it's blatantly obvious.
Instead of just writing a way more readable summary like
"Yamato looks at Momonosuke while remembering some words Momonosuke said to her/him earlier."
redon puts out a completely forced he/him festival where you have to think thrice which pronoun stands for whom:
"Yamato looks at Momonosuke as he remembers some words Momonosuke said to him when he was still a child."
That's just agenda bs and it has been going on for a while.
Guys like Artur actually admitted their mistake and went on. Redon is still causing annoyance/confusion with his nonsense.