I believe this reveals the essence of what happened here. Apple simply kept that same exact styling that they developed for Compact Mode and applied it to Separate Mode. The problem is that the use of a "hover state" to indicate "active state" doesn't make much sense from a UX standpoint, because
"hover state" assumes you're looking at the content. This is why the text contrast is reduced. That also explains why the "close icon [x]" appears; when you are hovering over content, your mouse is very nearby, so it makes sense to reveal actions you can take on that content.
You can see similar behavior when you mouse over the active tab in Compact Mode; Additional buttons reveal themselves only at that moment.
In summary, this is a simple (and clear) case of building a well-thought-out UX and UI spec, and then applying it to an interface that it was not designed for. That is why it appears to be shockingly substandard. I think Apple *can* fix this, but it is going to require that they create some amount of visual design language difference between Compact Mode and Separate Mode. The two cannot simply use the same spec.
EDIT: The user that originally clued me in on this was
@mazz0 in this post:
https://forums.macrumors.com/thread...ign-is-counterintuitive.2315360/post-30394675