IRDA is 1-to-1 and basically requires both users to do nothing else but use IRDA (including being within line-of-sight of each other). For a use case like talking to a family member a few seats over on a plane, IRDA won't help you.

Bitchat, at least in principle, doesn't require you to sacrifice the entirety of your phone to constantly run Bitchat in the foreground, and can work over larger distances (and with a mesh architecture which lets you reach people that you wouldn't normally be able to reach).

Writing an app that has the equivalent of 2000s-era functionality is a day of work at most. You don't even need to, you can achieve similar objectives via Airdrop / hotspot names (the latter being used as a loophole to communicate across prison cells). This is not what Bitchat is.