Welcome to YorkieTalk
I honestly would not go with that breeder because the info is not at all correct.
You can test whenever the pup is old enough to have blood drawn but it could be inconclusive before 4-6 months or so. Some breeders on YT would only test pups if there were major symptoms or after they are 6 months old because of the risk of doing a blood test on a small pup.
Any size Yorkie can have liver shunt and Yorkies are 36 times more likely to have one than all other breeds combined. If the parents have been tested and the lines are clear, some people wouldn't even worry about testing the pup. It is a good thing to do though.