Like, I get the potential scale is different. But this is some joke video, not someone trying to actually replace David Attenborough or something.
I think it's legal if you're the copyright holder, e.g. Disney putting dead actors in their movies did not need anybody's permission, nor does any derivative work such as videogames as they own the image right to the characters they play.
It would be interesting if they did a Planet Earth IV with Attenbrough's voice without his consent as David plays himself really...Not a fictional character.
What happens when you look or sound like someone (more) famous though?