There's 200k weekly downloads of node on npm. A million flutter devs means that there's a comparable amount of node devs to flutter. I'm not sure how else to estimate it, but there's 170k github stars of flutter compared to 100k node, and a roughly equal amount of forks.
Github shows 98% of code is "Shell" on top of that there are also many different package managers that people use: fnm, volta that are build with rust. You can also install nodejs using homebrew.
No. It downloads binaries from nodejs.org. There is no "official" canonical means of managing Node versions. It's just a grab bag of community tools like NVM.
The 2024 SO dev survey https://survey.stackoverflow.co/2024/technology/ has flutter at 10% and nodejs at 40%. Github language stats has JS about 10-15x the popularity of dart, but seeing as JS is more than just node it might pan out. https://madnight.github.io/githut/#/pull_requests/2024/1
So I guess looking at the stats 1M flutter devs is not an implausible number on its face.