“They think enough like mortals to know that people matter.” would be a reasonable way to rephrase that sentence. I agree that “thinking” is not normally a word I see used as a noun – i.e. “mortal thinking” is an adjective phrase in this case. So linguistically it DOES work, it’s just uncommon – which is probably what caused the feeling you describe.

I understood the original text’s meaning, even if the phrasing did feel a little awkward or unusual. Maybe on re-read Sage will feel the same and tweak the language, or default to leaving it as-is; author’s choice after all. However I think it’s helpful to respectfully share when you get this kind of reaction, so thank you for doing so! Outside perspectives can be valuable, and speaking up about what you see can encourage others to do the same.