"where people would add their ID to an enemy's medical room"
How is this done? I never heared about anything like that.

Yes, they can. This is to address espionage issues where people would add their ID to an enemy's medical room so they could spawn on it later. In a scenario like that, if you hack and repair an enemy's medical bay, they will know immediately that there is a problem and simply hack it back.

so how does this work in base gameplay, can people not just steal the medbay with grinding and welding?

I was not referring to a specific exploit. I was just saying "if someone manages to include their ID somehow ." There are multiple non-exploit ways this can happen. There are exploits that are not specific to doing this, but could enable this as well as other things.

has this exploit been patched yet?

On many servers you can only spawn at a spawn point that has your Steam ID listed in the custom data. This ensures that newly constructed spawn points have your Steam ID (and any others you want, such as your faction or alliance mates). It also ensures that only the IDs you provide are listed there. So if someone manages to include theirs in the spawn point through an exploit, this script ensures it will be removed.

Sorry I don't get it, what is it used for ?