It might be a common belief but I'd need to see some evidence, e.g. double blind testing, specific info about the masters that are used by Tidal and Qobuz.
if you have some tracks that you’re interested in i can sign up for a Qobuz trial and ask my partner to help me with a double blind test. (He knows nothing of this argument or what Tidal and Qobuz are). It would make an interesting thread on this forum, just FLAC files nothing else.