Not necessarily. They could split the video in advance, assuming the ads will always be at the same point. Even if not, they could still use the direct, unaltered source with a range. The big challenge would be keeping it all synced, which I think is safe to say that they will get right.
But even if it did need to be transcoded, YouTube automatically transcodes every single video uploaded, multiple times. They are clearly not afraid of it.
Will this help users on Bing with their number 1 desired search destination, Google?
Or their 7th most common destination, Bing?
Source: https://www.theverge.com/2021/10/1/22703263/google-lawyer-argues-bing-used-find-google-top-search-defaults