With the menubar, toolbar, and the tabbar, 106px of my total 800px height display gets to display non-content information, much of which is clutter when I'm trying to focus on the webpage. It's a whopping 13.3%! Most of this comes from the thick toolbar that Big Sur has started.
But since Apple won't be changing that thick toolbar (as we all know), the 30px vertical height (which translates to 3.6%) that I get by hiding the toolbar is precious. So I appreciate the new Safari 15 design. Really, the only problem I'm finding is the refresh button, which I'm like 99% sure will come back with all of this fuzz, and the other functionality in that (...) button needed multiple mouse clicks in Safari 14 anyway. Like... disabling the ad blocker required a long-click on the refresh button, it's now more discoverable.
Shifting address bars... I can see how that might make people freak out; Personally I've had zero problems, so YMMV.
About tab management – I can't disagree more than the article. Creating group of tabs is very much useful, it's much more helpful than having a group of windows each with different topics and prevents idle windows eating memory and CPU when only one window gets used for a long time.
I have five tab groups, one about my school, two on my personal hobbies, one on generic development-related information (including HN) and one on my work, each with 10~20 tabs. I'm guessing the writer doesn't use tabs pervasively – that's fine. But I would like to point out that it is not rarely efficient nor overall unconvincing. Thanks Apple for that tab group feature, I'm seriously getting a ton of mileage over it.