He seems to be a (former) independent contractor rather than an employee/representative, so I think this is more just misguided fanboyism rather than intentional astroturfing/reputation laundering.

Karat is probably the best/fairest you can do for purely algorithmic interviews, but I'm not sure they're even able to do anything else (the parent commenter's experience suggests not).

I would also consider it more of an exam than an interview - the process is very standardized and sterile, and obviously since it's being outsourced there is no way for the candidate to ask any questions about the company.

(disclaimer as well: went into the interviewer onboarding with them as a contractor but dropped out as I wasn't comfortable proctoring tests that I myself wasn't an expert at).