Currently actions-sync tool doesn't support the syncing the release assets.
This is tracking issue to see if more customer upvotes, then we will prioritise accordingly.

We have seen some customers wanted to sync the release assets as well into their GHES instance.
From technical standpoint, we need to make lot of effort to support granular asset downloads. looking at the current library we use (github.com/go-git/go-git) and assets are considered non-native to git and significant changes needed.

from github.com

from GHES, when synced, we don't see release assets