Technical error? A computer glitch doesn't create ads, someone wrote that code and the only error was that they weren't planning on rolling it out yet.
Exactly. Ads were created, the tooling was written into the game, and only then could a 'technical error' be responsible for activating the ads at the wrong time.
The ad is intentional. When it appeared is the issue. I've seen similar ads in multiple games when at the title screen.
Halo will pop an ad for new content, battle pass sales, etc. Hitman had ads for new content. NHL 23 showed ads for new content... All on the opening screens before getting in-game. I imagine the bug here is that the popup was triggered by an overlay or menu transition without excluding it during gameplay.
Also, it's not like it was an ad for Pepsi or something. It was an Assassin's Creed game showing an ad for an Assassin's Creed sale.
Technical error or "we accidentally turned on a new feature that we are still trying to work out how to get people to accept, let's accidentally turn it on and see how they react"