This was initially brought up during the Vergecast in which they spoke with Tony Goncalves, the AT&T executive in charge of HBO Max. They asked Goncalves if HBO max would count towards the user’s data cap, in which Goncalves claimed not to have the answer, but AT&T later followed up with a confirmation that it would not.
Apparently, this is due to a sponsored data system which allows companies to pay carriers such as AT&T to exclude themselves from data caps. In a way, it would be like the streaming company is paying for the user’s data. However, it should be noted that since AT&T owns HBO, they are basically paying themselves.
The bad news is that this is only for HBO Max, so if you’re using a different streaming service like Netflix or Disney+, then it will count towards your data cap, so you might want to keep an eye out for that.