Hmm, I'm not really understanding how it would be capped at 1,000,000; surely it should be non-negative up to 2,147,483,647 ?

Regardless, this is well defined by the Steam API to be a signed integer, and all valid values fall within 1 and 999999999 as outlined here:

Am I missing something?

Well I had an error where my steam itemdefid, was set to 3 275 882 830 (a workshop id) and it ran ok on steam's side, but when it got retrieved via the game it returned a negative number.
According to the documentation from steam the itemdefid should be below 1 000 000 if it's not a workshop item id, at least that's my interpretation from it:

So if i add a steam workshop item id, it should allow it, and should be bigger than the 1 000 000 and in this case it's about 3 thousand times bigger than the limit they mentioned.

Update:
From my understanding the "internal use" for steam, in this case, workshop is internal, so i'd assume it's "accepted"?

Okay yeah this is making a lot more sense to me!

I'm still a little conflicted just because this is how it is in the Steamworks SDK as well;

How are you going to/from the SteamItemDef_t? I ask because I /thought/ the 3275882830 that you have is actually a PublishedFileId_t -- but I can vaguely see how you ended up with it on the Inventory side of things with like a Curated Workshop.

Now though my worry is PublishedFileId_t is defined as typedef uint64 PublishedFileId_t; so this will become very exciting once the workshop gets up to ~4,294,967,295 ! 🙈