Language isn’t set in stone. If enough people are using one spelling, then that becomes a new spelling. A lot of spellings, words, phrases, and meanings from 100 years ago would be unrecognizable to you. People who pretend that the words they like best are the only correct ones are just being jerks for the sake of looking smart.
I mean, I found it in two pretty substantial dictionaries after a quick search: https://www.dictionary.com/browse/sike https://www.oed.com/search/dictionary/?scope=Entries&q=sike
Along with countless other dictionaries that cover slang and alternate spellings, but I figured you wouldn’t think that those counted.
But I will say that if being in a dictionary is a requirement for a word being part of the English language, you’re not accounting for a large portion of modern American/Internet vernacular. People who actually study language would never have this take, as languages are made up of the words people actually use, not the words that a board of folks decide to put into a list.