In my personal opinion, the lack of GTK4 a plus - that makes it lighter and easier to port. Bonus points for their choice being OpenGL. That is technically a minus on theming, but I feel like one does not typically theme games, which often need to have their own style.
I do concede that most people probably have GTK4 installed for something anyway, so if this application were written in GTk4, it most likely wouldn't take up extra space on their machine.
In addition, I don't like GTK4 due to client side decorations and those kinds of applications overall just tending to be more GNOME-oriented. Now I wouldn't call GTK4 the spawn of evil - I still use GTK4 applications when they're the best tool for job, especially when it comes to Upscalr or GNOME Clocks. It's just not my favorite GUI toolkit.