That's not what a health guarantee is for. A health guarantee helps protect you from genetic defects within the first 1-2 years as there is no way to test for these issues in an 8 week old pup. The puppy above has already gone through health testing which I just want to say is very expensive. So, there is technically no reason to offer a health guarantee.
The breeder is selling the dog at a significantly reduced price but would like to keep him on a co-own to use in the future. Again, you don't get compensated for that.
This dog was imported from Hungary. The cost and shipping alone on that puppy was thousands of dollars. He has trained him, housed, vetted and fed him for almost 2 yrs. so add in a couple more thousand. Then the breeder has done health testing which is a couple grand. I'm betting that breeder has close to $8,000 invested in that future stud prospect, so $2500 pet price is a steal on a co-own agreement.
No breeder offers a guarantee against your puppy getting a virus/ infection or injured. That is your responsibility as a pet owner.