SCALE will perform as good or better performance for SMB workloads and includes a newer version of Samba that rolls up many bug fixes and enhancements vs CORE 13.0. I’m not sure where you got such information.
That’s not an arbitrary distinction. Given the amount of money you’ve got invested in this question I’d say it’s important that you understand the difference between the connector and the protocol.
Do you foresee any issues with using Scale and 1PB storage being shared over Samba? I don’t have much experience with TrueNAS.
IXSystems sells storage systems that are greater than 1PB, and ZFS is the Zettabyte File System so I’d say from the perspective of the size there are no issues. However the implementation nuances from the hardware perspective of a system this large are not for the for the faint of heart.