Psmith o 'native' = non-OpenBSD.
'Spoofed' = ignored/written over?
But what does having an existing OBSD disklabel on the disk have to do with preserving the native partitions?
That paragraph is a bit confusing, esp. as the source only mentions "spoofed" once, and this in the context of (not) resizing non-FFS/non-SWAP partitions--see line 750.
My sense of this is that: if you have spoofed (ie. non-native) partitions already existing in an OpenBSD disklabel, then an automatic allocation (during upgrade/overwrite) will keep these partitions untouched--which makes sense because they're not OpenBSD-created partitions and outside the OpenBSD allocated disk range. Maybe native FFS partitions are preserved too, but, don't know how/why they would need to be "spoofed". Best to ask the OpenBSD folks.