Its not being argumentative its correcting possibly misleading or incorrect information. As you say you are a hobbyist, I would think you would have some caring in clarification and what is correct for your future knowledge. People make mistakes, all of us, no big deal but sometimes its how we handle our mistakes.
Whether you were given wrong info I don't know but the "older" NDM as you said, did not just disappear. Some nurseries may not be propagating it anymore but I guarantee that some definitely are. There are some growers who feel the inherent problems with the #4 do not make it worthy of propagating. Yes, #4 is a less vigorous grower but revisit the tree in 15-20 years and let me know the size. As I stated, less vigorous does not mean its a dwarf. And yes, there are a handful of sub-cultivars of NDM here in Florida, NDM Sai Tong being one.
As to Zills' "new" varieties, they are new to the "market" but the parent trees are not three years old either. The parent trees are well established trees grown from seed, some of which have been producing fruit for at least since 2009. The parent trees could very well be more than 10 years old. As you may know, the Zills have been experimenting with mangoes for a long time.