You don’t have ownership of UTXOs as you would in self-custodial Lightning if you’re running your own node or using Phoenix, Zeus, etc. Spark is built on bitcoin but you can’t just settle on-chain directly without trusting the Spark network operators.
Here’s how they explain it in their docs:
Spark operates under a “moment-in-time” trust model, meaning that trust is only required at the time of a transaction. As long as at least one (or a configurable threshold) of the Spark operators behaves honestly during a transfer, the system ensures perfect forward security. Even if operators are later compromised or act maliciously, they cannot retroactively compromise past transactions or take money from users.