About retrieving iLvl by scantip
 

For those items from timewraped instance or heirlooms, most of the gametooltip addon try to retrieve iLvl by creating a scantip. Same thing happens in libStub,
Why don't they retrieve all the item levels from this scantip thing?
They won't have to update the upgrade table anymore if do so.
What's the disadvantage of the scantip?

From what I understand, scanning tooltips on special items such as upgrades, timewarped, etc doesn't actually return the item level you are looking at on the normal tooltip.

Nope, all the items would return the right levels by scantip.
but they just retrieve timewarped and heirlooms by scantip, others by upgradeTable currently.

If you only supply itemID then you will get the plain version. These days there are modifiers in the item string that adds these labels like Timewalking or Mythic, item level upgrades, and so on.

I am sure if you provide the proper data to the tooltip, it will render the expected tooltip that you can scan. Note that it shows data based on your character and specialization, things like class trinkets will not show anything for you, if it's not your class. :P