YouTube has always sold inhouse, and the release (or perhaps the PR email) suggested this was the first time they’ve decided to go out of house.
I must confess, I don’t know what “YouTube audio ads” means, unless it is for a potential YouTube Music ad-supported tier. As far as I understand it, YouTube Main doesn’t have any audio ads - you can’t turn the screen off or play in background without paying for YouTube Premium.
I think any audio ads need to come from the audio ad budgets, which YouTube hasn’t had access to. Easier to get someone that does, to unlock those, rather than start a new internal unit. Plus competition between YouTube Main ads and YouTube Audio ads will be less destructive if one is sold by a third party.
SiriusXM is the largest podcast audio ad company in the US, as far as I can see. As ever, YouTube is focusing on a US strategy rather than a global one - SiriusXM has no sales capability outside the US at all.
(You’re in the audio of Podnews Daily today.)