IMO, GNOME 3's version of Adwaita was evocative of the old "Platinum" UI for Mac. Very clean and professional, and aged much better than Aqua for my money.
GNOME 3 Adwaita had some commonalities with Platinum, but in comparison it's a bit too bright in light mode and has much worse information density with how hard it leans into margins and padding.
In those ways I think Elementary's theme is a bit more similar to Platinum. It's not nearly as afraid of mid-grays and while it still has a lot of padding, it's less excessive than Adwaita's.
To be clear, I like that design style, but it is clearly true that it went "out of fashion" 10+ years ago.
[0]: https://en.m.wikipedia.org/wiki/Aqua_(user_interface)