There's no proof of that. In fact there's still no proof to my knowledge that nicotine itself is harmful - to the contrary, it may have neuroprotective properties.

> there's still no proof to my knowledge that nicotine itself is harmful

Nicotine itself is a deadly poison. MSDS: https://fscimage.fishersci.com/msds/96314.htm

"May be fatal if absorbed through the skin"

The LD50 in rats for caffeine is < 200mg/kg and this MSDS has a neat skull & crossbones under the words "toxic if swallowed":

https://www.fishersci.com/shop/msdsproxy?productName=O172850...

...also for reference caffeine seems to be more than an order of magnitude more toxic than pure ethyl alcohol by weight.