Finally got to doing this. Maybe there's variance here, or some process change. On my 2021 Rebel, the frame mount has a welded nut.
Contacted Core, and they said the hardware kit (I requested the full kit) should have two sizes of bolts based on model year... the usual/larger ones + smaller ones that will bolt up to the welded nut w/o modification.
My hardware kit had no smaller bolts...but if you are ordering, check what you have and be specific in what you request since they do have out of box solutions.
That said, I don't want to wait on more shipping so just going to drill out the nut and use what I got. Need to remove the rear wheelwell liners anyway to re-torque rear shocks.