Nicotine itself may not be a carcinogen, but its metabolites are.
Nicotine itself has been demonstrated to be a tumor promoter by way of increasing tumor cell division in lung cancer and inhibiting apoptosis.
Quoting chemical highlight 25-1 from "Organic Chemistry" 6th edition by Vollhardt & Schore:
Nicotine appears to play a dual contributory role, because its metabolites are outright carcinogens and because the parent system itself, while not causing cancer, is a tumor promoter.
The metabolic pathway has as the initial step the N-nitrosation of the azacyclopentane (pyrrolidine) nitrogen. Oxidation and ring opening (compare Chemical Highlight 21-3) then take place, giving a mixture of two N-nitrosodialkanamines (N-nitrosamines), each of which is a
known powerful carcinogen.
Upon protonation of the oxygen in the nitroso group,
these substances become reactive alkylating agents, capable
of transferring methyl groups to nucleophilic sites in biological molecules such as DNA, as shown below. The diazohydroxide that remains decomposes through a diazonium ion to a carbocation, which may inflict additional molecular
damage (Section 21-10).
The lipid pneumonia outbreak was a thing exclusively associated with THC vapes, which are an illegal but widespread cottage (garage) industry where one summer, one of the thousands of manufacturer-enthusiasts made a forum post about the innovation of maybe using vitamin E acetate as a thickener. Experiments were performed, positive results were obtained, and products went out to distributors. The hazard to heavy users (perhaps for manufacturers with poor blending practices, we don't know) who showed up in the ER, was recognized within a month or two, and everybody immediately stopped using vitamin E acetate as a thickener. It took most of a year of panic for the last of that summer's merchandise to percolate through the supply chain.
The outbreak was initially hard for users to trace in particular because of how brands worked in that (again, moderately illegal) industry - a "brand" was basically a paper label/bag production line shipped in the clear from a printer, to hundreds of individual manufacturers, who negotiated their own distribution. Conclusions like "Mellow Mallow Blurple is a safe brand, I tested it" ended up being invalid.
There was a lot of fog of war at the time, and a lot of things were reported in the media that were inaccurate (or reported to doctors that were inaccurate, this being documentation of illegal drug use). This is my conclusion about what actually went on, aides by a number of articles in the tech, health, and especially cannabis media. Eg:
All you need to defend a Wikipedia claim staying in the article is a journalist writing something, and journalists with zero idea of what they were talking about outnumbered informed writers a thousand to one.