In this context, a runtime model is the conceptual/execution layer that decides what the system means and how it behaves at runtime.

A sync engine is the replication layer that decides how state moves between peers/devices.

Put differently:

Layer	Main question	Responsibility
Runtime model	“What exists, what does it mean, and how is it used?”	Documents, links, facts, app semantics, subscriptions, queries, permissions, live state
Sync engine	“How do peers converge on the same data?”	Replication, fetching, publishing, conflict handling, availability, peer-to-peer transport

So a runtime model may say:

“There is a document, it has blocks, links, comments, metadata, capabilities, and live derived views.”

The sync engine says:

“Here is how those document changes, comments, blobs, and signatures get discovered, transferred, verified, and stored across nodes.”

A good boundary is:

Runtime model = product/application semantics

Sync engine = distributed systems plumbing

They overlap because the runtime needs synced data, and sync needs to know what data objects exist. But they should not be the same thing: the runtime should be able to express behavior without being tightly coupled to a particular transport or replication strategy.