meaning - What does "may" mean?

“Well, Voldemort’s going to try other ways of coming back, isn’t he? I mean, he hasn’t gone, has he?”
“No, Harry, he has not. He is still out there somewhere, perhaps looking for another body to share… not being truly alive, he cannot be killed. He left Quirrell to die; he shows just as little mercy to his followers as his enemies. Nevertheless, Harry, while you may only have delayed his return to power, it will merely take someone else who is prepared to fight what seems a losing battle next time — and if he is delayed again, and again, why, he may never return to power.” (Harry Potter and the Sorcerer's Stone)

The second may has the meaning of 2 below, while the first may seems to be 1. But it is within a while clause that has the meaning of although, it’s not clear for me. What does the first may mean?

OALD

1. used to say that something is possible


2. used when admitting that something is true before introducing another point, argument, etc.

Answer

You have it reversed.

The first use of may admits a counter-argument before introducing a main point. The second use expresses a possibility. On the one hand, Harry only delayed his return (not fully defeated him), on the other hand it's possible he'll never return to power.

Notice that in the 2nd definition, the word may is "used when admitting...". May by itself isn't enough to create this meaning, it's only used as part of the idiom. The While here is simply another part of the construction that admits one thing before introducing another.

While you may not believe me, I still stand by my story.