I get that you are feeling slint is not GPL, but I do not understand where that feeling comes from.

I think it's because the for-profit nature of the company may not create an actual community of FOSS enthusiasts around it. So if something were to happen to the business side of Slint-the-company, there would not be a strong community with known leaders and vision to save the situation. It's not like this is guaranteed for FOSS projects, but it's much more likely there. Single-person FOSS projects are scary to me for this reason as well.

That is the biggest factor for me. (There are other important factors, too.)