So, after a quick literature search:
There is no published evidence of cast iron seasoning carrying any special carcinogenic elements, aside from those carried by any kind of high-heat cooking in any kind of cookware. If there's a danger from cast iron, it's folks who season their cast iron in an unventilated kitchen, since the burnt oil fumes are not good for you, and the fact that folks sometimes heat cast iron to higher heats than they would other cookware, just because they can.
For that matter, quality modern nonstick cookware in many countries also lacks known carcinogens. Teflon coatings used to consist of PTFE and PFOA, of which only the PFOA is a known cancer risk. For that reason, most manufacturers in the US and Europe have made PFOA-free nonstick cookware since 2013. PTFE, the substance that actually resists stickiness, is so inert and harmless it's used to coat medical instruments. Like cast iron, the main danger is if you overheat the pan, which does release substances that are bad for your lungs (also ruins the pan).
Heck, you can even today choose from an array of pans that have a non-PTFE nonstick coating.
Given all this, there are many reasons for you to chose cast iron vs teflon cookware (or even carbon steel, or many other options), but cancer risk shouldn't be one of the reasons.