I don't know where he is getting his information. It's not clear whether he's just speculating or whether he's getting this from the team. Rex didn't say anything indicating this at his press conference. Can we all just agree this is a horrible idea, though? Nothing these two do is remotely as valuable as covering receivers. There would be no reason to risk getting them hurt doing something else. The team can sign a guy off the street to do that. There are unemployed guys who could do a good job.
Cromartie wouldn't work anyway considering his tackling issues. Risking Revis out there might even be a less attractive option since he's a better cover guy than Cromartie.