This means that switchdev drivers won't ever have to treat a HOST_MDB
notification any differently than a PORT_MDB notification

No, they need to treat it very differently. 

A PORT_MDB says that frames for a group should be sent out that port.
So it probably needs to iterate all the ports in the bridge and add a
match/action to each port saying frames coming in for that group
should be sent out the port listed in the PORT_MDB.

A HOST_MDB say that frames for a group coming in from the port listed
in the HOST_MDB must be sent to the host. The match/action applies
directly to the port, other ports are not involved.

	 Andrew