Yes, I don't know why, the SMP support is far from "rudimentary". I don't think we ever had a Giant-lock, and our scheduler is pretty good, too (runqueue-based, O(1) w.r.t. threads, O(log N) with respect to cores, fully utilizes ACPI topology information.)
Er ... what? Where are you getting this information? Haiku has been in development since 2002, and has SMP support since ... 2003/2004?
NUMA support is a little shaky, but I know users who have run Haiku on 32-core Ryzens with no issues. So we definitely have SMP support.