I have some doubts about this translation, specifically the word "mass". It is related to roman rite liturgy, from words "ite, missa est" at the end. Byzantine missionaries wouldnt bring roman mass but byzantine liturgy, which is neither called mass nor it uses such wording.
If this is really mistake of translators then I doubt they could translate legal terms well enough.
by the way, Slavs of Great Moravia were mostly christianized peacefully. They accepted eastern liturgy, because eastern concept is closer to common human, more understandable, and eastern missionaries traditionally understood that people have different cultures and different approach.
Mostly only conservative nobility sticked to paganism because they used it to maintain their power over poorer class of society.