in the short term, people will work crazy hours to hit a date if it's the difference between a 7 figure bonus and getting fired. if the estimate is based on devs working reasonable hours, that's a lot of slack built in. I'm sure they hit the date more often than not in your scenario, provided they control most of the dependencies, but it's not a sustainable approach for delivering features.

"provided they control most of the dependencies" is a pretty important caveat by the way. many times I've seen people get the rug pulled out from under them by partner teams at the last second. it doesn't matter how clever you are or how hard you work. if you depend on something owned by a team far away in the org chart, they can always blow up your project with little consequence.