You are skeptical that someone who has what appears to be otherwise great grammar and spelling is not from the US... because of one mistake? That everyone and their grandmothers has made at least once in their lives?

Have you seen the 45th president? Have you seen the average American?

Are you actually serious right now?

Considering how often Americans write "could of" instead of "could have" I believe there is a correlation between poor grammar and being from the US.

If you judge people based off of grammar online then the US just lost 99% of its population.

Oh please. Move on. The internet is full of bad grammar by native speakers. It's the nature of speech. Who speaks the Queen's English?

bro, I have family from the southeast US who write sentences that don't look like english. lol. Chill. The US is a big place with people who have varying degrees of DGAF when it comes to spelling/grammar.

There/their/they're is a very common mistake among people born and raised in the US, same as too/to/two.

Your third line is entirely unnecessary.

Lots of UK native people make the same mistake, never mind the US. Give him a break.

English is not the official language of the US. :^)