Spelt is actually a type of grain, and if you are talking about the grain, it is correct. If you are talking about to spell (a word) in the past, both "spelt" and "spelled" are correct, but "spelled" the preferred spelling.
Oh dear, not according to the Cambridge Dictionary:
spell (FORM WORDS) verb [I or T] spelled or UK AND
AUSTRALIAN ENGLISH ALSO spelt, spelled or UK AND
AUSTRALIAN ENGLISH ALSO spelt
to form a word or words with the letters in the correct order
Modern dictionaries only give the spelling of words, not their usage in every day language.
Oh dear again: how "modern" is an online dictionary? Cambridge again:
to do the same thing as someone else:
When one airline reduces its prices, the rest soon follow suit.
- I kid you not, that is the example given!!
English is a living language, inevitably it will grow in different directions given certain inputs, but that's not what we're discussing here is it? We're discussing the name of a legal entity (Qantas Airways Limited); QANTAS is an acronym as IBM is. Marketing types doubtless register and link various spellings of urls to hit their own website; that doesn't mean that the spelling is correct.