²⁰⁹Bi decay to ²⁰⁵Tl is alpha decay — the bismuth nucleus splits into an alpha particle (helium) and a thallium nucleus. I think you’d be hard pressed to find a nuclear physicist or engineer who calls that “fission”.

Here’s the Wikipedia article on fission. It’s a rather different process:

https://en.m.wikipedia.org/wiki/Nuclear_fission