Pretty sure in most/all the cases mentioned, they were available as extensions long before Zen had them. In particular, both Zen's and Firefox's built-in vertical tabs feel like a bit of a downgrade from Tree Style Tabs (which I continue to use on Firefox, and also used on Zen last time I tried it).

Depends on what one values. I tried TST and Sidebery among other extensions for Firefox, but their "bolted on" feeling always bothered me. That can be somewhat fixed with userChrome mods, but those tend to be fragile and it's not unusual for them break with updates, and even with the mods they're not as integrated as native vertical tabs.

For my personal usage I find the deep nesting of TST to get in the way more than it helps and so the simpler model of Arc/Zen where nesting only happens intentionally is nice.

Might be true, but that was not the point. The comparision was Firefox and Zen, not Firefox, Zen and whatever else is out there :>