The more I think about it, they might be planning for a trade to occur very shortly. Like tonight.
Cominsky has to be on the 53-man for a single day to be eligible to return - not sure if it's 24 hours or effectively two days though. If their WR was going to be a waiver wire addition, they could have kept Peko on the 53-man, designated Cominsky to the IR tomorrow, and had their waiver wire addition replace Cominsky. The open spot is only needed if a move was going to happen in that single day where Cominsky is rostered.
Though the salary implications of Peko not being on the initial 53-man matters too, so it could be that.