What evidence is there of this?
I'm not aware of any hard evidence. I consider it "almost certain" because the
Gesta Hungarorum claims that the Hungarians, immediately upon their arrival, encountered there Romanians (along with others). Such chronicles are far from reliable, but it does indicate one thing - that by the late XII century, the Romanian presence north of the Danube was old enough to be
considered ancient by their Hungarian neighbors. The Primary Chronicle of Rus, compiled around 1113, makes a similar claim.