Everything in it is evidenced by as much research as they can get.
https://www.surgeongeneral.gov/library/reports/50-years-of-p...
I agree that it's a bad idea to take nicotine in any form unless you need to, but it seems that nicotine is one of the less harmful parts of tobacco.
For chewing tobacco it's probably not the nicotine that's causing cancer, but all the other stuff.
But more to the point, there was little reason to believe it would cause cancer in the first place, compared to any other random compound. Its association with tobacco is a wash, because other compounds, like aldehydes, are completely sufficient to explain the carcinogenicity of smoking tobacco.
For analysis of nicotine as such, you can refer to the known side-effects of nicotine dermal patches; they're well researched, approved by FDA, etc - the place of application can get skin irritation and itchiness; overdose of nicotine can get dizziness, headache, rapid heartbeat, and nausea (same as for too much nicotine through other means), but they're not known to cause cancer or other significant damage despite being used by very many people over very long time.
Just like a light hit on the head would increase awareness, but increasing the force and repetition of the hits on the head is not well advised, when the effect wears off after overuse - to say nothing about the immediate effect of getting hit.
The perceived short term benefits are only secondary effects. And with regards to nicotine, all the arguments in favor are to be treated as extraordinary claims.
I wonder how the testing methodology works.
To the first order of approximation, nicotine addiction is bad for your health because it causes you to burn and inhale plant matter, the common delivery methods of nicotine are horribly unhealthy but if you're talking about nicotine as such (for example, through a transdermal patch) then it doesn't really have significant adverse health effects.