Hnath was not the interviewer. The interviewer, Steve Cossan, is credited in the program.
As I've said in some other recent threads: I don't object to Hnath being eligible for a Tony for his work (which he will be). I just think it's a different kind of work than actually writing the text of a play, and I think that reflects on the limitations of the Tony categories as they exist. Again, I said this in the recent thread about the Best Play category - the Tony categories are only designed to recognize theatrical art within a very narrow, constricted box of what theatre can actually be.
Regarding Grey Gardens: I don't think that's an apt comparison, considering the entire 1st act of the show was original text. But I do think you're right to point out that many musical books stick very closely to their source material, and yet are put in the same category as completely original works. Again, one isn't better than the other, but it just feels like a different kind of work.
In theory, I think the Tonys would benefit from having separate categories for original vs. adapated scripts like the Oscars, but there probably aren't enough shows to fill those categories - especially on the musical side. Which opens up a whole other can of worms about how the elitism surrounding Broadway actually renders the awards LESS meaningful, because it makes the pool of contenders so small compared to every other major awards show.