This is close to being correct, but it isn't. Server side ad insertion is possible, and the tech already exists. See, your browser by default sends cookies to the server with every web request, no JS required. Simply* add the ad videos to the list of chunks.

*By "simply" I mean it's technically possible, I know it's not simple to implement.

It's also already done for many podcasts, they'll have changing/seasonal ads for all episodes they're hosting. Which is confusing if you're listening to a 10 year old episode and get some new ad.