Li Ion battery reconditioning?

Some Lead Acid batteries and Nickel-Cadmium cells can be reconditioned.

I have never seen such claims for Lithium based batteries.

Lithium based batteries age even when they're not used (as you found out). Some batteries suffer more from this than others ! Their chemical structure changes in a way that cannot be restored. So in my opinion the marketing guys talked nonsense.

To be absolutely sure ask the battery manufacturer about the shelf life of their batteries. They have an interest in selling you new batteries of course so they might be inclined to say you need new ones. So I'd ask them how long their batteries last in storage and how they need to be stored.

This is a myth. NiMH batteries I believe can be reconditioned but Li-ion cannot. We store our cells in a controlled environment at a specific state of charge after they are manufactured before being installed in batteries. I would guess that the reason for the capacity being lowered isn't due to the fact that they were stored for 3 years, but most likely because they weren't properly monitored.