Well, if you have solid technical experience, but are being very nervous and quiet, and a recruiter tells you straight to face that "culture fit is more important", then the reason for failing 30/30 interviews seems fairly clear.
Why don't you A/B test yourself? Take double dose of your favorite anxiolytic and dress more casually to next interview and see what happens. If you fail again then it's 31/31 and not a big loss.
The key is to figure out WHY you don't do well in technical interviews, and then fix the root cause.
Do you known/guess whether the problem is the actual technical part, or everything surrounding it, which makes you appear as "sub-optimal cultural fit", e.g. person that is so nervous that they make everybody else feel uncomfortable too.
Can anybody from recent batches comment on the extent of actual advice and mentoring received during the program?
E.g. do you typically work closely with your advisor(s) on near daily basis, or is this more like booking a meeting with your university professor occasionally, when you need to discuss a particular topic?
Why don't you A/B test yourself? Take double dose of your favorite anxiolytic and dress more casually to next interview and see what happens. If you fail again then it's 31/31 and not a big loss.