Once you've tinkered with the item, it is yours.
Yep. They 'bought' it when the decided to disassemble the product. Now it's their responsibility, if they wish, to get it repaired.
From the maker's website (translated from Russian):
8. Is there a guarantee on your knives?
Our knives extend "lifetime" guarantee. Any manufacturing defect is corrected under warranty. We reserve the right to refuse warranty if intervention in the design of the knife.
Perhaps it could be sent to Russia for repair by the original maker?