Keep in mind that Teheran still had 5 years of control left.
Had he not signed this extension, here is a hypothetical salary progression with that term and AAV:
500k
500k
3M
6M
10M
12.4M (FA)
The Braves basically guarantee him that ^, which includes one FA year of control, and they also get a 12M team option on the 7th year. So they kind of just guarantee Teheran close to what he would've received in arbitration anyway in order to gamble 12.4M on Teheran for 2019, and then they get a good team option for 2020.
It's decent for Atlanta and it's how teams do business now, but it still has tangible downside. Think of what happened with Romero.