What i could find: No!
Yes, synth uses jails for its builders, but those are exact mirrors of the host-system
Contrary to poudriere, where you can specify the Target OS-version when creating the basejail, from which then the builders are copied from.
So, with poudriere, say, you have a host with 12.1, you create a basejail with/for 11.4, and then use that basejail to build your packages for 11.4
At least: That's how i understood it.
Might be wrong....