I personally know that some old uses of "y" replace "thorn" to give a theta sound, yes. :-D

Yes, false etymologies piss me off too. I still can't see the word "isle" without getting steamed at some long-dead and unknown (to me anyway) British prescriptive etymologists who added the "s".

But if you're contradicting someone's claim, sources are useful, because otherwise it's your word against theirs!

And is what you are saying really so? This article doesn't seem to support it: https://en.wikipedia.org/wiki/Thou

?