No, they were pretty lightly equipped. At least all sources say that. They did know how to build siege devices though (attacking Thessaloniki).
This is terrain I'm talking about. Imagine a horse struggling to walk on those rocks, and then hundreds of people spring out of bushes and trees or even rocks and start throwing spears at you. That is how wars were waged here.
Or in dense ancient pine forests on hills.
In any case, it all comes down to single event or series of events the result of which could have be decided by many factors, and we have no source on anything. But if we forget the terrain and Croats and Serbs, generally I believe that it's entirely possible for lightly armed and armored infantry to defeat cavalry. It takes good leader, seasoned men and a bit of planning. You surely don't want to find yourself standing in front of cavalry archers on open grass field. And whoever survived that situation once, or knows how it would end, would find another way to fight them.
Finally, I don't believe in great arrival of Croats and Serbs. Mixed 4 centuries old legends. They came with the rest of the Slavs, were among dozens other tribes, but gradually over centuries conquered, assimilated most of Slavs and pre-Slavs around them, and still exist to speak of it. But they must have fought the Avars at one time or the other. Everyone fought everyone. And in every legend there is some truth, so in this one also. Won't know how much until someone invents the time machine, but it wouldn't matter then because he'll screw up history. That Idiot, I'll have to relearn everything again.
P.S. That circle means you have received reputation points. that means someone likes what you posted. That's the green + all the way down below avatar, next to report button. Though I don't know if you can see it, you may not have enough posts to give reputation.