I read your post and a lot of what you said I can relate. I did almost completely anaerobic training prior to UA. Came from a background of martial arts kick boxing and and boxing where you trained all out all the time.
I discovered Maffetone and pretty much diagnosed myself as aerobic deficient. Following his pace I could hardly walk up a steep hill without going over the threashold. I got the breath exchange test done at the local university and came with numbers a bit higher than Maffetone.
I used those numbers and followed the UA program. After about a year I can hike up the same hill with 30 lbs on my back, a faster pace, and hardly ever going over aerobic threshold. My point is this stuff works.
It could be that the test result were off. I would recommend the heart rate drift test as it is accurate and can be repeated easily. I doubt you’re going to do the gas exchange test every month.
I know that UA has had good results with people that have a highly developed anaerobic system and poor aerobic system, which seems like yourself it seems.