It's more the former. I'm assuming though that Background Security updates are basically the same thing as "Rapid Security Responses" was, which on the Mac I can recall being used once, 13.3.1(a) released the same day as 13.4 as an RSR.

Basically, the amount of stuff Apple can realistically change on the fly without restiching an entirely new system volume snapshot into place is quite small, so unless the stars align it can't be used.

See: https://khronokernel.com/macos/2023/04/18/RSR.html