Yes, removed games are not really removed, just hidden. And for the reader here, we are not talking about hiding in the client. If you go to support page of game at https://help.steampowered.com/en/wizard/HelpWithGame and click "I want to permanently remove this game from my account", then its like you wouldn't own the game anymore.
Not many know you can go to same page and restore the license. It's noted that you get the same license of game than before, not a new license. I don't know why Steam has the permanent removal, if its not permanent at all. But now you know; you can restore. And that is what the user in this post is talking about.
Edit: I thought about why that maybe. It makes sense to keep the license bound to an account, so the key cannot be activated again by someone else. Otherwise people could sell their activated keys like that.
There are reasons for why that is possible, for better or worse.
To reverse malicious actions that may have been taken by other people that got access to the account for any reason for example.
There may even be legal reasons for why that is the case. Licensing law can be quite complicated.
And then there is the fact that if it's hidden it's effectively gone anyways and that may actually be what people actually care about. Not whether or not they technically still have that game or not. People wanting a clean library is a thing and it ties into the concept that is the right to be forgotten.
Also, even if it was permanent, it would still be something like a permanently_removed set to TRUE in a database. License keys probably are one of those things no company truly ever deletes from their records.
Yet... Ubisoft did go and "remove" the keys for the crew when they shut that one down. No one but them can really say if they actually deleted them or not. though I have my doubts with how that company has acted in recent years.
I feel like the main reason would be money laundering prevention. It's slightly harder to create new accounts than it would be to have one account repeatedly buy, remove, and repeat for new licenses.