German 'einer/der Meining sein' means to have an opinion about something, so you aren't the opinion itself, but see something a certain way. In the sentence 'Sie sind verschiedener Meinung' they disagree about something. 'einer/derselben Meinung sein' means they agree. I don't think your translation reflects that, does it?
You mean, as to say: this is thesis a, this is thesis b. Those are various/different opinions? You would say: Sie/Das sind verschiedene Meinungen. At very first, you need the plural, unlike in duo's example. But that's all context-dependend, one would especially use other words than 'Meinung' then, probably.
What you are actually missing is the grammatical case. 'verschiedene Meinungen' is a predicate noun, the ones you check for equivalence, as you already tried. Here they are. Sie sind - what are they? - verschiedene Meinungen. You use the nominative in this case. When it's the dative case, like in the example, it refers to the idiom, that's the only and not even very obvious difference in this example. I hope I'm not completely mistaken here, somewhere. That's really nothing you think about every day.
Thanks. You're right, I missed the case indicator in the adjective. I'm still not attuned to looking to those for meaning. I've been caught that way before; this example might set that need in my head. And because this example doesn't translate word-for-word literally, it suggests another possible difference between German and English grammatical structures--one I should watch for. Cool! Thanks again.