[phenkdo]

It's not Buddhism per se, The Zoroastrians (religion of pre-islamic Persia) also practice this by leaving the bodies in "towers of silence" for birds of prey to devour them.

[yakubin]

Yes. I think the author of the paragraph wrote about Buddhism in the context of Tibetans. It isn't applicable to Mongols as well.

[selimthegrim]

Mongols and Tibetans probably got it from them given their original location in Bactria/Margiana