Not all extensions that offer to hide tabs use Mozilla's tabs.hide() API.
Does use the API.
When hidden tabs are restored, not all are loaded. This is good for performance.
When first added to a clean Firefox profile, the first move of a tab to a group is followed by the standard alert, from Firefox, that an extension has hidden a tab.
Probably does not use the API. When hidden tabs are restored, each one is loaded.
I don't recall seeing an alert from Firefox after hiding tabs.