I have also been thinking about it but we don't know what context its been used in. JKR has used a great choice for the character to say this Gildory>> lol as we won't believe anything he says if he said the world was round we would question it.
But he does say that he's taking credit for other wizards/Witches work so maybe some one has. But also we don't know if this werewolf was a werewolf like it could be like a Bill type werewolf not a werewolf but with werewolf tendicies. so maybe this quote relates to Bill and he could be cured. i find it interesting that Greyback is mentioned a lot in the last books and when he is talking to DD; DD says something about Greyback attacking when not in werewolf form and Greyback says he does so it isnt impossible that the cured wolf could be like Bill.
But i don't think he will change he excepts himself now and i think DD would have been able to do it. But spells are always being made so it is possible i hope so.

who knows it could be like the muggle world people thought it was impossible to cure Maleria or TB untill a few years and i'm sure the magical medical world is more accomplished then the muggle medicine world. So that Witch/Wizard could if been a medical doctor thing

making a medical break thorugh.
BJB