I helped Spark's devs design their protocol under contract. I can confirm OP's description is roughly accurate, although there's another layer of complexity on top of the basic statechains described here.

A statechain UTXO by itself is not super ergonomic because you have to transfer all of it or none of it. Spark wanted to design a system where you can subdivide statecoins up into smaller units using a tree of off-chain transactions, by sharding the keys that control the locked UTXO. More info here. The cryptography behind it is really cool, but not described very thoroughly in the docs. Maybe they're reworking the protocol since I left? IDK