Instead of rudeness, it would be useful to get some links to information about about "thank you" really being a valid answer here.
So far, the only thing I've found, which is unfortunately about British English rather than the American English we primarily use here, suggest it wouldn't be valid: https://dictionary.cambridge.org/grammar/british-grammar/please-and-thank-you.
Note that I am not looking for "I am a native speaker and I say so" type responses here, but specifically links to credible resources. Thanks in advance!
We get lots of native speaker feedback of speakers from both languages that is plain wrong. Being a native speaker certainly doesn't mean one is infallible. I've actually learned some stuff about Finnish from working on this course and looking up answers for users -- stuff that I apparently managed not to learn for several decades before that, despite having had writing good Finnish as an important part of my job for the last decade or more.
Here, just as in all debates, it's good to be able to show some evidence to back up one's opinions. Unfortunately, with Finnish being a small language, there aren't always sources in English that go into enough depth (to be useful for everyone here, we of course can consult them anyway). However, for English I would expect there to be ample evidence to point to. I still haven't had anyone point me to anything supporting this interpretation of thank you.
And if you're specifically asking about my reply to your previous message, I can only repeat: this is not about the Finnish, but about the English.
Is this the type of thing you're looking for? https://www.merriam-webster.com/dictionary/yes%2C%20thanks
Where do we draw the line between "correctness" and communicative competence? I cannot imagine a single situation where saying "yes, thank you" instead of "yes please," would be understood to be rude OR incorrect... after a certain point, isn't that what matters and why we learn languages in the first place-- to be understood?