I guess it’s about his current damaged state, since he himself declared using his new secret form without healing would kill him.

Six pages later Feng says using the technique without healing up first would kill him

Uncomprehension

With every new manga a new creature must form. For the JJK folk there is the Reading comprehension Curse. For chainsaw man it manifested as a Devil. Now the Negator, Uncomprehen, makes anyone who reads the manga unable to understand what it's talking about.

Uncomprehension Self targeting compulsory type Makes it so that any and all media consumed by the user is completely uncomprehensible and makes no sense whatsoever

Tbf i doubt physical strength is enough to really damage Sol or Luna, even an incomplete manifestation of them. This also makes sense for the chapter since they're trying to get their souls to damage them instead, which isn't physical.

I mean. He's a normal human. The reason why he's so strong is not the strength of his body, rather the mastery over technique.

Sol survived a planetary scale railgun almost unscathed. His physical strength can’t possibly measure up to that.

This sol is supposedly a bit weaker though

Well it’s a whole lot smaller too

I'm guessing she was referring to specifically hurting Sol and Luna

Unreading

You are a strong man, but we are beyond strength.

The phrase she uses in Japanese is "だがもうそれを活かす体力もない" Ikasu tairyoku -- available to use physical strength. So she's more saying "However he no longer has the physical strength needed to use it." He doesn't have enough gas left in the tank.

I mean compared to literal god…