Most scammers aren't that smart. The grammatical mistakes just happen to work well for them as a filter.
I’ve heard this a lot, but is it actually true?
Unlikely. I believe it started as a hypothesis to explain the many spelling errors in scam mails, but I don't think this have ever been confirmed to be intentional.
Hanlons Razor is probably appropriate here.
(Of course, there are other reasons why would one spot yet ignore grammatical mistakes - sometimes it is because of the personal situation they are in, sometimes it's of empathy towards non-native English speakers)