The AI F pricing policy appears to apply to more than just ORD -- FRA, or even EWR -- FRA. There are special F fares valid only on AI 126/127 and AI144/191 (numbers change 10/25, but the routes remain generally the same -- the Chicago flight goes to BOM instead of DEL).

These F fares are 'way lower than even AA's DUPIND fares -- as, for example, US$4056 all in one way from ORD to either BOM or DEL. Cheaper still on a round trip basis.

So, the question OP asks -- which bears repeating -- is quite right: is there a catch? Or is the service in F actually significantly better than in, say, Y, where reports on Skytrax generally give AI really negative marks?

There are very, very few reports on any forums or blogs about AI service in F...