This means, "of all Poles in the world, many live in Moravia". I'm sure they meant to say "there are many Poles in Moravia".
Yes, I agree that your English sentence makes more sense.
On the other hand, as you probably would agree, the English DL uses here is in fact what the Czech original says.
žije looks singular
mnoho, málo, hodně, trochu, více, méně,... require singular
I never realized this! So “dvere” requires plural but “more than one” singular. And I thought I really had a handle of all of its.