It could be a mistake with the way the game is interfacing with the Steam API (IIRC, some other games have created problems along those lines in the past); the launcher is a separate application and shouldn't have any reason to utilize the API, so it working as usual is expected. If they were truly going to add DRM, they'd just add Denuvo.Durante, would the reports that some people had of being able to boot to the launcher in Offline Mode, but then having it crash when attempting to start the game itself be consistent with the diagnosis that this is a bug? I personally don't see how that specifically would be possible when it's a bug, I do understand how the launcher/game refusing to start at all in Offline Mode/without an internet connection in Online Mode could be a bug
Even being a bug, it's certainly not guaranteed that Square will ever fix it (I wouldn't put any money on it).