Does it perform a full store-wide sync?
No. SKU Stock Sync does not perform a full store-wide inventory sync.
The app does not scan every SKU and force all matching inventory to update on demand. Instead, it processes SKU/location groups when they become due because of inventory changes or routine checks.
Manual sync does not force-sync your whole store. It processes work that is already waiting.
If there are no due groups or setup items waiting, there may be nothing for manual sync to process.