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.

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.