Does this apply only to supplemental Black cards? In other words, can you have more than two supplemental cardholders with Platinum (or lower) cards on the same account?
Yes and yes.
It explains that the changes are for supplemental black cards (both in the fee increase and the maximum amount of 2 additional cards) and specifically notes that no changes are being made to supplemental platinums or golds (and then reinterates what the charges are for them, e.g. the first 3 platinums, each additional, golds, etc).