Maybe that's changed, but a couple months ago we tested it on vanilla Ubuntu in the team at $dayjob and we could see everything. Nothing was grayed out.
Sounds like it works a little differently on Linux.
To be honest, I'm actually surprised and impressed that they support screen share in the Linux version because of how many different flavors of i.e. WM there are in the wild.
Zoom on Mac and/or windows seems to black out other windows if they appear on top of the target window. I've seen that happen a few times so definitely does happen, just perhaps not on Linux.
