I am usually the first person to scorn Urban Dictionary (for the same reasons I assume you hold), but I can't deny that that first post contains the same opinion I believe.
There are a few unconfirmed theories (based on sensible statements) about where the Proto-Germanic speakers came from, and certainly the general consensus is Denmark / Germany is where they originated before branching into East (Gothic), North (Old Norse) and West (English, Dutch etc)
Every book on Old English has the disclaimer that we will never truly know the pronunciation, but a lot of good work has gone into reconstructing the best guesses, the most credible theories, and others have even gone back further to postulate in Proto languages like Germanic, or even further to Proto-Indo-European, historical linguistics isn't something which has a lot of physical evidence available, this is just one aspect, reconstruction of vowels / consonants through sound change laws and evidence in comparative theory from other related languages.
Indeed, I don't suppose many of us do realize that. We think much of the language might have come from Old French, Old German, Latin, Greek, Arabic, Portugese ... etc. Not forgetting Norse and Norman French, of course.
People are more than welcome to think those things, but factually all wrong of course.
But there are those who enjoy reading about etymologies and historical linguistics who
do have an idea of general patterns and a clearer idea of what the linguistic truth is.
I'm not saying I'm right, I'm certainly not saying anyone is wrong, but there is certainly a credible / linguistically sound explanation for the origin of that particular word, consonant loss after a Danish borrowing, vowel lengthening because the word would have been a light monosyllable, and a more prominent northern usage. If the Vikings can give us our plural pronouns why is it so hard to assume it can't have happened with a word that means
thank you?