Maintainer of Material for MkDocs here. Correct! The Sponsorware strategy is an alternative to dual-licensing which works fairly well for our project. If you have a successful Open Source project that is a pain killer and product-market fit, this strategy might also work for your project.
https://squidfunk.github.io/mkdocs-material/insiders/
Roughly 10xed their sponsorship revenue in ~1 year.