Colloquialism isn't the right word. They were thinking something close to "misnomer".
Also, I think your issue is that the rather bare terminology of the concept of "media with accessibility difficulties" doesn't allow for certain nuances in meaning that can lead to confusion.
You say that a piece of media that is truly, genuinely gone forever is completely different from one whose access is simply restricted by a corporate institution? That is true. I see your point.
But what of media whose continued existence itself is uncertain? How would that fit in? No one can find it, maybe every copy's been destroyed, maybe it never existed in the first place, so it may not even be real. After all, tree in the woods, and all that. Wouldn't that count as effectively lost?
Now, that kind of thing counts as lost, since no one can find it. You could say that media with restricted access is NOT lost, since we know where it is, it's just hidden. Thing is, though, "lost" the word can also apply in the context of "money was lost." You know where it went, you just no longer have access to it.
Wouldn't that also work for media? You just no longer have access to it, but it's still there, you just can't touch it.
But I understand your frustrations.
All copies of X were lost in a fire.
Y is an early work of an esteemed creator only screened to the public once. Reception was bad and it was left to rot in the creators' computer, a testament to their embarrassment.
Current terminology would call both of these lost media, but saying that Y is lost media gives the impression that it, like X, can never be found again.
There is nothing in the lexicon that provides you with an adequate semantic distinction. I totally understand. Some media is more lost than others, and that can make for a huge difference.
I guess you'll just have to wait and see how the concept will evolve. Maybe we'll be lucky and get a proper neologism to describe the difference