Yes, the CCU-DEL flight is a domestic (former IC) flight, and so the DEL-BOM-JFK experience is not germane.

However, all kinds of peculiar things happen with AI. My last flight with them, ~15 years ago, was on a domestic leg DEL-CCU. (This was AI, not IC.) Domestic and international (pre-Customs/immigration) passengers were mixed on the same plane. We arrived in the international terminal in CCU and exited through immigration waving our boarding passes and muttering "domestic, domestic..."

In other words, it is not impossible that domestic and international (post-Customs/immigration) passengers are carried together on the CCU-DEL flight, now that the former IC flight is an AI flight.