they did originally use yak butter, and still do, but increasingly do use butter made from cow milk, since it is more available now. Maybe you saw a case of that.
Update: Just searched again and found a few more interesting links. Also, they (Tibetans) pour the butter tea into their tsampa, also a staple food among them:
https://en.wikipedia.org/wiki/Butter_tea
they did originally use yak butter, and still do, but increasingly do use butter made from cow milk, since it is more available now. Maybe you saw a case of that.
Update: Just searched again and found a few more interesting links. Also, they (Tibetans) pour the butter tea into their tsampa, also a staple food among them:
https://www.eater.com/2016/8/25/12624068/butter-coffee-tea-t...
http://www.teafoodhistory.com/tibetan_tea.html
http://www.npr.org/sections/thesalt/2015/05/05/404435137/tea...
https://en.wikipedia.org/wiki/Tsampa