Steam Inventory Scanner gets data via Steam Endpoint that returns the Inventory Data.
This Endpoint is public and the Application will see only what you would see if you actually looked at the given Inventory.
There are Two Endpoints available, referred to as "Old" and "New".
The difference between the two comes in the way they return Data to the Application, but more importantly in the Speed and limitations. Which one is used depends on the Information provided during the Profile Creation or Link used during the URL Scan.
Old EndPoint is accessed via the Inventory URL and is Slower.
New EndPoint is accessed via different URL but Requires SteamID64.
Therefore we recommend including SteamID64 or using SteamID64 during URL Scans, because this will automatically mark Profile for new EndPoint and give faster results, especially on really large inventories.
Steam Inventory Scanner can't provide SteamID64 Automatically'
You will need to provide SteamID64 yourself, as Steam Inventory Scanner doesn't use Steamworks API and has no means of getting it from Profile URL.