But you do get all code under GPL, which you seem to agree is a freedom preserving choice. That does make Slint free software and as save to use as any other free software project out there.
The story is a bit different when you want to contribute to slint. Slint is not a even playing field for contributions. The company has an advantage in that if can use all contributions as MIT and you can only use the companies changes under GPL, commercial, or royalty free terms.
As a contributor you need to decide whether you contribute to the slint repository, or maybe write an add-on library which you are free to license as you see fit.