AS/400s actually support 3270 terminals. Apps natively speak 5250, but OS/400 has a built-in 3270/5250 conversion layer. IBM wanted to sell AS/400s to their mainframe customers who had already invested heavily in 3270, insisting people had two terminals on each desk (3270 and 5250) was a non-starter, hence the emulation layer.
Makes less sense now just about everyone uses emulators rather than physical terminals, especially when you can get emulators which speak both - but still supported anyway. You can set up your IBM i to accept TN3270 as well as TN5250.
That said, people confusing 5250 for 3270 is more common than people actually using 3270 with an AS/400, so this is probably a case of the former rather than the later
I was at my insurance agent a few weeks ago and they still had a 3270 emulator in use.