The FBI can't arrest, but they can work with enforcement agencies that do have jurisdiction (local, (Indian) state, (Indian) national) and provide resources and information.
The story itself talks about one of the senior local officials who won't talk much about the investigation, but "he will describe the raid, in loving, cinematic detail: How at 10 p.m., after the last of the call center staff had arrived for the night shift, 200 police officers streamed up the main staircase, blocking every exit and detaining all 700 people who worked inside." That's not 200 FBI agents, that's 200 local officers.
I haven't read the entire story yet, but part of the reason that the whistleblowers contacted the FTC (and through them the FBI) may be corruption - if they went through local channels and picked the wrong person, that person might have simply gone back to the leaders of this with their hand out and the information on the whistleblowers. The FBI may not have jurisdiction, but they also don't have a reputation for accepting bribes.