Laos, Burma and Thailand all have old tea trees. In Laos they're in the Phongsali area, not far east of Yiwu in China and as a consequence, it's not uncommon for Phongsali tea to end up in Yiwu tea. I have seen Burmese da shu cha being pressed in Jinghong, and used to know the owner of a tea plantation outside Chiang Rai, who also had (access to) old tea trees in the mountains outside Chiang Rai, but never got to taste it.
There's no reason to suppose that Puer made from old/ancient tea trees in neighbouring countries is inferior per se. Obviously, different histories in different places will have affected tea cultivation and the trees and also, there are many different micro-climates. Cultivation practices and tea making skills are likely to cause the biggest differences.
Someone I know here told me a while back that they had been to Phongsali and got fresh leaves from which they made tea themselves. They reckoned there was nothing wrong with it.
We had some Burmese tuo around the shop somewhere that someone had given us - can't find them now - 'Jungle Tea' or something. As I can remember, it wasn't a huge disappointment.
Plenty of those old KMT folks went to Shan state when they left Xishuangbanna, and must have taken their tea making knowledge with them, though perhaps they turned their hands to more profitable crops.
If I understand the phrase correctly, 'border tea' refers to tea that was made specifically for export to neighbouring countries or border regions, as opposed to tea that was grown and made in those countries, but maybe that's wrong?