He did it once before at a rally. Here, towards the end when he says "I'm not being solicitous, I'm being deadly earnest." Supercilious?

"solicitous" used to describe the behavior of someone who's insincere, who's just trying to get something or get you to buy something: a solicitous salesman, for example.

But he should use a different word.

Or, better yet, just stress that he's being sincere.

Or, still better, just express his ideas and feelings. I think people can hear his sincerity.

I think most people understand "solicitous" to mean caring and concerned. If he's using it to describe someone who is trying to get you to buy something, so he can say he's not that—well, it's too obscure. And head-scratching. It's poor communication.