This blog post focuses on some superficialities of how their DE's apps will look slightly different. That's understandable for first impressions.
I do hope these superficialities don't have all of System76's focus, as they're a dime a dozen in Linux DEs. Even the category of "we kind of look like Gnome, but with more familiar workflows" is oversaturated amongst Linux desktops (Budgie, Xfce, Cinnamon, MATE, Elementary/Pantheon, even "Gnome+extensions" are all in this category to various degrees). I suppose one distinguishing factor that Cosmic has is a strong Wayland focus, which is still missing from nearly all Gtk based alternatives.
System76 with Pop_OS! has an opportunity to tackle topics head on like "we can make fractional scaling work somewhat decently across all apps" (IIUC currently requires shipping a forked XWayland, unfortunately), "we can make trackpads the best they can be" (requires shipping some forked libinput related things IIUC) or "we can make font rendering best we can make it". The actual desktop environment stuff I'd be interested in.
A desktop environment needs more vision than shipping the same old Linux desktop problems with some other apps. I really hope System76 can make an effort there. They're trying to make their paycheck depend more on their own Linux desktop's success, and that I can only encourage.