It is an anagram in English, but not in all languages, and not even in all English speaking areas, as in some "Father Christmas" is the more popular term. Satan is not an anagram for Father Christmas.
So it doesn't work universally even in English. But in other languages it doesn't work at all. Spanish and Portuguese spell the name of Satan as Satanás (better matching the New Testament spelling, Σατανᾶς (Satanas), than English does, actually), which is clearly not an anagram of Santa. But you might point out that other languages do use the Satan spelling. But looking at some of those, I see no anagram for Santa. In German, Santa Claus is Weihnachtsmann, which pretty obviously is not an anagram for Satan. In French, it's Père Noël, again not an anagram for Satan. In Dutch, it's Kerstman, once again not an anagram for Satan. Your argument relies on a spelling similarity in one language that doesn't even work for everyone in that language, and then ignoring the rest of the languages.