> The fact that a service exists to make you better at interviews implies that interviews don't evaluate competency.

I don't think so - it just means that your competency can be hidden, and it's possible to make it more visible to the interviewer.

For an extreme example if I go into an interview and remain mute, then I won't pass. If you teach me that I have to speak then I would do better in the interview. My technical competency hasn't changed between the two interviews.