Anyone can restore a device with iTunes or finder. When you restore a device, Find My link is broken and the device is untraceable. Find My and Activation lock are separate systems. Erasing a device by restoring it breaks Find My remote tracking, but does not disassociate the device hardware ID from the AppleID so Activation Lock remains in effect.
So whnen someone sees their stolen device drop off of Find My, but it is still listed under their AppleID as a trusted device, then that tells them someone erased their device by restoring it. The owner cannot remote tracking it any longer, but the thief also cannot activate that erased device and use it.
Note that Activation Lock goes into effect the moment an owner signs their device into iCloud, whether or not they enable and use find my.