That is strange. But it's also within the rules. The non-APFA crews are authorized to fly internationally, but not beyond MIA into the USA. It's possible that it was a last-minute crew substitution.

As far as the equipment qualification goes, I believe that 763 and 752 crews can cross-train fairly easily on the other equipment. It is surprising that the SCL-based crews would be 752 qualified since only the 763 currently flies to/from SCL. However, it's not out of the realm of possibility that some of the crewmembers choose to re-certify in the 752 in order to be able to pick up an occasional extra short-haul trip while they are at MIA awaiting their return to SCL. A whole bunch of possibilities come to mind: CCS, BOG, PTY, SJO, etc. Come to think of it, perhaps some of the SCL-based crews are AB6 qualified as well.