Nicotine is in fact highly protective against Parkinsons. The effect is not small!

Smoking has declined by at least half in the last fifty years in the United States. Also not a small effect!

"Very plausible" here means "someone should really do a study where they correlate for lower levels of nicotine use and see if there's anything left to explain".

I can't imagine anyone would see "very plausible" and think the explanation should be accepted without investigation. That's just not what that phrase means.