Nvasec DDG ATP does not block cross-app tracking. It does block known third-party trackers in apps. It occupies the VPN slot to do so.
Sandboxing is something you have in all Android devices. The access control it gives is independent of what the DDG app does. GrapheneOS does some hardening on the sandbox and exposes two additional permissions (sensors and network).
DDG's approach has a few shortcomings:
it occupies the VPN slot, which might be better used for a VPN or Orbot to hinder IP tracking.
it only recognizes known third-party trackers. It doesn't prevent unknown trackers or first-party tracking.