Yeah, a bit stupid, they even showcase the browser on macs only (guess they like the clean look). It's a GTK app, Gnome3 apps do look similar to osX apps. The full screen button though, it is very out of place. It is problably the result of using it on Elementary OS (where it is de default browser), a Linux distro that aims to emulate osX. [0]

[0] https://elementary.io/