No matter where the Japanese bombers land after toasting the UK battleship and transports–Siam or Kwangsi–they are vulnerable to an aerial UK counterattack.
The problem (if you consider there to be one) is that Japan can also land plenty of fighters / tac bombers to defend these bombers as well, so a careful Axis player can negate the effectiveness of this potential Allied counter.
I don’t think it’s realistic to assume that the playtesters miscalculated, or failed to notice the J1 bomber range: that’s the first thing they probably checked for. It’s more likely that they thought that the cost of putting all of Japan’s bombers in that area of the map in the first turn would have some longer-term strategic cost that balances out that Axis advantage (e.g., this move makes US stronger, richer, faster)… although the playtesters themselves would be the best ones to testify to that. Yoper?