It may be a hidden tab from a technical POV, but from a user's POV it is not hidden, making this a confusing user experience. This should not be the case especially since the tab manager is now also enabled by default, making this user interface element more prominent than before. Firefox View is in fact different than really hidden tabs, therefore it should be treated differently.
It's also not clear to me why this access point is needed at all. It's pinned to the tab strip by default AND there is still another entry point with the menu bar.
It's also not clear to me why the context menu with options like "pin", "clone", "mute" or "share" is relevant for Firefox View. Most options make sense for websites but not really for Firefox View.
But if you think it should be included in the tab manager, then it should not be listed as hidden tab at least because that's just not true (again: from the user's POV; it's not about technical implemention details). Just add it at the first level of the tab manager. Then it still takes space that I would like to use for one more tab instead but at least it's not wrong and therefore confusing.