Metaverse is an undefined buzzword that means whatever they want to changing each time to fit the nothing they are selling you.
What most agree on it being is a fucking hubworld. that's it it's been done most MMO's have them as you said (either serverwide or instance based) however what they don't want you to know is the metaverse as they vapidly describe it has also already been done a place that is everything all at once.
Decentraland exists that is what "the metaverse" is accidentally describing it's self as a massive project with none of the talent skill vision money or competence to make it feasible but because people are idiots other people make money off it anyway.
Why can't we just use fucking virtual world is that not special enough for these money grubbing fuckwits. Stop making shiny new words to sell your nothing to people.