。
She talks about it as if she knew everything.
Literal
She [topic-は] that [about-について] all [object-を] [knows-知っている] [as-if-かのように] talks.
~かのように is the simile pattern for hypothetical comparisons — 'as if X.' Built from question か + nominalizer の + simile ように, it frames the embedded clause as a counterfactual or impressionistic comparison rather than a fact. Often paired with まるで ('exactly, just as') for emphasis. Here it's used without まるで but still carries the same edge: she is talking in a manner suggesting she knows all, which the speaker subtly questions or doubts. The pattern can be biting, admiring, or descriptive depending on context — here it leans toward subtly critical.