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.