Again, it's about organization (and I guess utilization).
Someone I know has 3-5 of every tool, which is in some ways fine, but if you need to use them, you have to dig them out form under a Pile of other stuff. He also has parts to cars he has never, nor will ever, own, which again are not tidy and can be hard to find. To me, that's a hoarder.
Some people accuse me of being a hoarder too which can be frustrating. I believe there is a difference. Every tool I have is neatly organized in cabinets or on shelves. If you need a tool, you just go to that spot and get that tool. It may seem cluttered to the unknowing because. Lot of stuff may be on one shelf, but everything on that shelf is in a specific spot and is the same kind of tool.
I do have a few piles of what appears to be unorganized things, but everything that is in an unorganized pile is organized to some degree. For example, I have an x2 pile and I own a x2. I have a pile for all the stock parts to the Superjet I currently own. I could sell those Superjet parts, except parted out ski sold at stock is worth more than a limited, and when my msd failed, I just got the stock cdi off the shelf and went right back to riding.
So again, the defintion of a hoarder, in my opinion, is about the level of organization (and to some degree the utilization).
Edit: I am hoarding a gp1200 motor that is so destroyed it is scrap, but I have a good reason for that too. I'm hoping to put a gp1200 motor and 155 pump into a x2. I'm holding onto it because I believe someday I will be abler I make it happen (I've mocked it up and I am closely approaching the needed skill level to get 'dr done). Lol.