As far as I know, power has nothing like the SRAT that tells us, at
boot, which memory is hotpluggable.
On pseries we have the ibm,dynamic-memory device tree property, which
can contain ranges of memory that are not yet "assigned to the
partition" - ie. can be hotplugged later.
So in general that statement is not true.
But I think you're focused on bare-metal, in which case you might be
right. But that doesn't mean we couldn't have a similar property, if
skiboot/hostboot knew what the ranges of memory were going to be.