Why does the `nixbld` group have a GID of 30000?

Always thought of Nix worker users in nixbld as system users, and so I thought nixbld would have a GID that reflects that (i.e., below 1000). (Not that there is any difference in any way, just curious why this number was chosen.)

On Mac:

$ dscacheutil -q group | grep -A 3 'name: nixbld'

name: nixbld                                                                                                                                                                                                                                                                  
password: *                                                                                                                                                                                                                                                                   
gid: 30000                                                                                                                                                                                                                                                                    
users: _nixbld1 _nixbld2 [..]

On Ubuntu:

$ getent group | grep nixbld

nixbld:x:30000:nixbld1,nixbld2, [..]

Unless Eelco weighs in or there are preserved discussions from the time, I imagine the closest to an answer is:

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I think a better question is we still don’t have cgroups isolated builds?