As described in the article one employee upsold $6k in extra products, but when she quit she was charged the full amount of the training bill.
This would seem to indicate these financial punishments/ threats don’t reflect anything the employer is actually losing, and the employee actions potentially have no impact on the cost.
Hard to really be informed as to what you are agreeing to when it seems to be an entirely arbitrary outcome.
I don’t think you can voluntarily / knowledgeably agree to a completely unknown sum.