Two separate issues. Pan-CJK fonts like Noto solve a problem that arises from the fact that many CJK characters are only ever used in certain locales. Since most CJK fonts are made for a given locale, they tend not to include any of the (many) CJK characters never used in that locale. Hence, if you render mixed CJK text in (say) a Japanese font, any characters that don't appear in Japanese won't render at all. That's what (I believe) this comment refers to.

The Han Unification problem arises from the inverse case - characters that are used in several languages but rendered differently depending on locale[0]. For those characters, they'll render even without a pan-CJK font, but the problem is they'll render in a way that's not appropriate for their locale.

[0] Another way to phrase this would be "distinct characters which share a code point becaus Unicode mistakenly thinks they're a single character whose rendering differs by locale". The difference is basically subjective.