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.
So to be a GPL project you need to be able to trust the project to release all future releases under GPL in addition to having released existing code under GPL?
I do not like this approach:
First of, you need a crystal ball to decide whether a project is GPL or not: Some projects managed to pull off a license change before, just by asking devs whether they are ok and replacing code from devs that did not agree. Its rare, but it happens, so checking for CLAs, copyright assignments, ..., is not enough.
Secondly this definition excludes lots of projects that release their source code under GPL, including the GNU project. They ask for copyright assignment, both to defend the GPL license, as well as to be able to relicense when weaknesses in the current licenses are found. I give you that GNU is probably way more trustworthy wrt. not changing away from the free software spirit than some random company.