>Isn’t this a bit short sighted? So if someone has a wide vocabulary and uses proper grammar, you mistrust them by default?

Yes, people, in general, do.

https://www.youtube.com/watch?v=k_gjWlW0kRs

I'd say, not "people in general" but people form other socioeconomic strata. This guy is not talking like us, suspicious. He talks in an elaborate and thought-through manner, not simply, so, he's not candid, double suspicious!

I'm personally suspicious of anyone using the word candid.