Is the setting OFF/ON of the security flag different from enabling/disabling of fastboot?
S-OFF is mainly found in HTC devices, and is a separate security mechanism whereas the /system directory cannot be mounted RW across boots.

It also restricts other features.

If S-OFF is enabled, fastboot can't be disabled, as fastboot would have to be enabled to turn S off. As such, fastboot would most likely be enabled, unless it was re-locked in some way (which would in turn lock S to ON).