English Translation
She speaks as if she knew all about it.
“ https://www.lawlessfrench.com/grammar/imperfect/”
See the section: French imperfect after si
The imperfect is often used with si, in which case it’s not a past tense. Instead, it expresses something that is unknown or unreal in the present or future.
So that is why the imperfect is used here after “comme si” even though there is uncetainty about whether she knows all about it or not.