He's right.
It will be at least a year or more.
The way it works is this. They've announced their intent to buy. Now they have to submit it to the FCC to get regulatory approval for the merger.
Inside of that submission is going to be a HUGE list of properties they're going to divest (both on the TMO and ATT side) as part of the merger. Most of these are overlapping area's where one or the other already has superior service and they're going to dump off the overlap to keep it from being a monopoly, among other things.
Most likely the FCC will reject that, and give them some more stuff to divest. They'll counter propose, and it will get approved.
Then they'll actually complete the merger a short time later as far as "we now own TMO", but neither company will change... yet. They will operate completely independent and a "merger rollout" will be planned, and market by market they will bring customers who have not been divested into ATT's system. This doesn't happen at once, but over a period of time.
They will then put up those Divested area's into auction, and companies will buy them and start a similar process to bring those customers (when an area is divested both the network and ALL customers in that market area go with it to the new company) into their systems.
I spent over 5yrs in the cellular industry, and the company I worked for at that point both bought a company and was purchased by another so I've seen it from both sides.
It's a long process.