Just to things i can think of
1) this 30 minute 10 km runner probably was mark allen who did this test in the form of a few times a mile with rest? This tends to give faster mile times at a low heart rate to.
Also to note is that if it was mark allen he was ? a triathlete who rarely used speed work and was training for an 8 hour event. So his 10 km time might not have been his true track fitness.
So if that is the case Maffetone might still be right. But you are right to that for a competitive distance runner that doesn't make sense.
2) If you listen to the man he says that if your times tend to stay at the same level after improving a while it might be time to work faster then MAF.
Also he says when you get very fit you can alternate days or do this in the form of interval work. Again not to unreasonable. What me means is that you need to work a lot at that heart rate or below in order to build a base. And that running over it might ruin your base training. He also says that this rule of 180 - age is a general rule. when he tested people he looked at many things.
And yes i think some of it can be applied to professional runners. And i think runners like Priscilla Welch used this in order to perform at world class level at older age.