Learn English – When the contraction can’t be used

I think you're overanalysing this one. Obviously native speakers often make "slips" like this, especially in casual conversation or Q&A website comment posts.

In the case of X mistaken for Y, I imagine almost every competent native speaker would say for is "correct", but many if not most wouldn't see anything seriously "wrong" with as. A (probably, much smaller) number might not see much wrong with with.

This is an example of a comparatively minor slip. On a site like ELL, for example, many users are native speakers, and many aren't. As it happens, we have contextual information for the specific comment that sparked this question, making it highly likely that the writer is in fact a native speaker. But even if there were no such evidence, the fact of the usage itself would be no real reason to suppose otherwise.

This shouldn't be taken as a general principle for all "misused" prepositions, etc. Some errors are far less likely from native speakers, and far more likely to be seen as "giveaways" indicating "non-native".

If you are one of those people who defer to etymology, then 'either' comes from the Old English ǽghwæðer, "each of two" (i.e. "both"). By the 13th century, the meaning had shifted to its present sense of "one of two". So the word has a history of gradual change. Shakespeare wrote "They say there is divinity in odd numbers, either in nativity, chance, or death" (The Merry Wives of Windsor" c.1597) (an odd number of items, naturally). The "one of two" meaning is relaxed, according to a number of dictionaries, when introducing a list (like the food items in the question), but tends to be more strictly applied when using 'either' as an adjective - "I'll marry either girl, Mary or Jane", or a pronoun - "either [of them] will do". There is a similar story for "neither". The safest advice is to stick to "either of two", and use e.g. "one of" for more than two items.

Either (Grammarphobia)
Either (Etymonline)