Someone using to vs too is not misspelling a word; they don't know the difference between two words. That's a grammar mistake. Would you say this is a misspelling?
This is a misunderstanding of how language works. The written form is derived from the spoken form and not the other way around. Using the wrong letters for words that are phonetically identical in your dialect is a simple transcription error, not a failure to understand the difference between the words.
"to" vs "too" are close enough that could be either a spelling mistake or a grammar mistake. If they get it wrong every time then it's a grammar mistake, if they get it wrong only once, then it was probably a spelling mistake.
This is not a misspelling. A person who would write this sentence does not know the difference between there and their. They do not think that their is spelt there.