He had a full trade clause with the Reds because he was a 5/10 (5 years consecutive years with 1 club and 10 years overall in the league) guy which is a universal no trade clause for every player that qualifies with that. By agreeing to go to the Braves he is no longer a 5/10 guy. He can be traded.
That's my understanding as well. Even if true, there's always a chance we agreed not to trade him during the season, but I doubt he would be upset over a trade to someone more in contention later in the year. And we can always move him to 3B or let him be a bench bat/utility guy once Albies is up.