I was in Playa Del Carmen 24 years ago. At that time, there were Army soldiers with machine guns at the Cancun airport entrance.
There was a whole lot of nothing until you got to Playa Del Carmen. We stayed in a hotel just on the north end of town. There were about 5 big resort hotels on the south side of Playa. I understand that has all changed dramatically now.
Yes, we saw soldiers there and at the Mexico City airport too. That whole trip i got the feeling the authorities were very concerned about protecting their cash cows—the tourists. The local people were friendly, and I seldom felt in danger (except on the trip to Chitzen Itza). I doubt they feel as protective of the gringos now.