The difference here is formerly tickets were issued by BA and thus were seen as travel agent tickets via GDS where the legacy rules applied and free seat assignment within reason was possible

Since the launch of rewards.aerlingus.com the tickets are issued by EI and EI has full visibility on the tickets, though strictly speaking these is no difference as the are still issued by a IATA travel agent (who can only sell from the X/U buckets)

So if you want free seat assignments book via BAEC...