Spelling it as "tilda" just reflects the fact that a reduced vowel in English may be spelled in any number of different ways (and "e" is particularly unlikely); we can assume that superkuh is more familiar with the word orally than in writing.
https://en.wikipedia.org/wiki/Tilde
This includes lots of detailed discussion of the mark under the name "tilde", including names of Unicode characters that include this term. Wiktionary also views it as an English noun borrowed from Spanish.
Sure, but it's not live in the same way that most words are. It doesn't refer to anything in an English speaker's normal experience; you could easily go your whole life without ever using the word.
Regardless of the symbol's function in different domains, the symbol's name remains 'tilde'.
Maybe you don't use the word much but that really has nothing to do with the price of fish.
The word is in conventional English dictionaries, like Oxford, Cambridge, and Merriam-Webster. As those sources will tell you, it's also used to describe the mark "used when writing some languages".
As such, the idea that it's somehow not a word in English is just silly.
No, it definitely has a name. Tilde. Heck, it's even the name used for the ~ symbol in Unicode, which famously names all characters with upper-cased English words. Just because it's not used in English orthography doesn't mean that there isn't a name for it, even if it's merely a borrowing.
> If you were studying Greek, you'd call it a "circumflex".
I suppose I would. Somewhat off-topic, though.
> Spelling it as "tilda" just reflects the fact that a reduced vowel in English may be spelled in any number of different ways (and "e" is particularly unlikely)
Or, as GP explained, they just misspelt it.
Responding to a later comment:
> you could easily go your whole life without ever using the word
That doesn't invalidate the word's existence in the language.