You are technically correct: Slint is free software. You can get Slint as GPL or commercial terms -- or the royalty free license. The latter lets you do whatever you want anywhere with the exception of "embedded" (this exception makes is not open source).
When you contribute to any MIT license project you are in the same situation: Your code will be redistributed by some company under different license terms. That's the point of MIT & Co. You contribute MIT code to a project, the project releases its code under MIT, and a company consumes the project and restricts its use. Slint is just cutting out the middle step here.
Disclaimer: I work for Slint and appreciate being paid for contributing to open source software. I also appreciate Slint being free software.