Nope. You've certainly got the right idea. Interoperability metric is measured by how easily 2 different blockchains can communicate or pass operations to eachother.
A side chain exists generally as an addition to functions of the main blockchain. Some sidechains have their own token/coin where others like the one I'm building rely solely on their parent chains native coin to operate.
The HSC will have a TON of tokens on it in time I think and people migrate their HE, BnB and ETH tokens over to the ERC-20 compliant HSC chain. The whole "Atomic swap" think you hear about on Ethereum is a buzzword they are using to explain interoperability at the moment.
Think of it as sort of a gateway into allowing 2 blockchains (or multiple) that speak different languages and have different code and tokens to be able to swap all of these elements back and fourth. That is at it's core what Interoperability is, creating systems that are blockchain agnostic and allow for all sorts of different blockchain offerings to be utilized or accepted by it.
RE: The Importance of Interoperability Between Different Block Chains, Moving Forward as the Age of Mass Adoption Begins to Materialize