I really think that it isn't, and there are several reasons for this.
Who's Who's head in hybrid looks the exact same as it did in full beast form, which might indicate that he is an awakened zoan. What if the true power of an awakened zoan isn't merely an increase in stamina and regeneration, but rather the abiliity to adapt to their environments? Essentially, every time an awakened zoan is hit by a technique, their body adapts to gain a certain resistance to it. The ultimate version of this is kaido, who has taken so many blows that his skin has become near impenetrable.
Basically, Who's Who could not only come back, he could come back stronger.
- Oda never drew Who's Who with his eyes whited out(or alternatively with his mask broken, similar to what he did with Doffy and his glasses)
- Who's Who is an ancient zoan and one of the strongest tobi roppo. If Ulti could come back after getting KO'd twice, first by Yamato and then by big mom(which literally blasted a hole through her), this shouldn't be all it takes to take out Who's Who
- Who's Who likely didn't go all out. He is portrayed to be Lucci's rival, with him having a similar fruit and showing similar rokushiki(such as flying shigan, and spots). There are still three major techniques of Lucci's which he is yet to show, those being Rokuogan, Life return, and kami-e.
Who's Who's head in hybrid looks the exact same as it did in full beast form, which might indicate that he is an awakened zoan. What if the true power of an awakened zoan isn't merely an increase in stamina and regeneration, but rather the abiliity to adapt to their environments? Essentially, every time an awakened zoan is hit by a technique, their body adapts to gain a certain resistance to it. The ultimate version of this is kaido, who has taken so many blows that his skin has become near impenetrable.
Basically, Who's Who could not only come back, he could come back stronger.