Could just be the cement. It looks like he's trying to write "normally" but the resistance of the cement (which yes, shouldn't be resistant as much) could just be throwing his hand off.
He only "shakes" while he's doing the writing, the tweet embedded shows this during the zoom-in, I doubt it's Parkinson's as Mike J. Fox had more an "uncontrollable" shake that was really apparent even before his diagnosis.
I haven't seen the Ellen interview, but I'm saying this is the internet making a big deal out of something before a confirmation.