I said nothing of the sort, and have no idea where you got that idea. All I said was that marketing claims are separate from the contract.
However, this thread is clearly not interested in any actual exchange of ideas or information, so I will no longer be taking part. Go ahead and downvote.
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.