I think this is where you and I are talking past each other. I'm not saying that multiple videos shouldn't be allowed. That's not the problem here.
The problem is that Safari has a mechanism to ensure that the user wants a video to play. That mechanism looks for some UI action on the user's part before it will allow a site to launch the video player. With this new multiple-video feature, that mechanism is now broken. It'll say, "Hey you want to play this video? Yeah, ok, I will allow it, and any other video the site wants to spam you with now."
That italicized part is the bug. It shouldn't assume the UI action applies to an arbitrary number of separate videos.
The video player is fine. That's the design choice. The Safari code not accounting for that is the bug.
"Every time a try to click this link, my browser crashes!" <--- that sounds like a bug.
"I'm able to create a webpage that exploits a user interaction to create weird behavior" <---- not a bug! if a problem exists it is within the realm of User Interface Design and not software design or anything within the code itself. The design choices may cause a need to rework the code, but that doesn't make a bug magically appear in the code.
> Okay, so it may not be a software bug at all, but I'm gonna move these goalposts and insist this is a product design bug, or something.
No they aren't there is a button that the user clicks that runs code to play multiple overlapping videos. This serves no conceivable purpose and can cause the browser to crash, it is a bug.
The reason it works is the code is run from a user action, the problem is after the first video play the browser should no longer consider the subsequent plays a user action, or it should only play the last video and cleanup the now overlapped previous video.