Based on the time between release and the beta. Betas nowadays start like a week before full release but they’ve already had it locked in for the past month. Alphas are generally feature complete when released to the public sure they’re buggy but they’re generally not that bad which is generally the hallmark of a beta.
Evolve for example I played the big alpha there were bug fixes and balance changes but it was feature complete
Blizzard IIRC actually did have an extended testing phase for Overwatch
Once it’s fully released it isn’t a beta.
They do have a definition you can call it a beta if you want but that doesn’t make it a beta.