That's also how I understand it. It requires a very specific and uncommon set of circumstances, but it could leak info to someone observing.

But also I think the main issue "pes10k" has is that it is adding a new attack vector in a backwards-incompatible way. So a sensitive website which is doing these JS lazy-loading things was "safe" last week, but now can potentially leak information in a way it couldn't have with earlier chrome versions.

In practice though it's hard (for me) to think of a website working this way. I understand the idea of a network admin sending the CEO a link with the text-fragment "cancer" in it. And if that fires off related requests unusually, I can assume it auto-scrolled to "cancer", and the CEO has cancer and sell all my shares. But what sort of website will dynamically contain the word "cancer" for patients that have cancer and is also doing fancy lazy-loading stuff? I'm not a security expert though and I know people can are much cleverer than me when it comes to exploits.

You're very right, what is the chance that this set of circumstances occur anywhere on the web? It seems like the person describing the scenario was just pulling a theory out of thin air in hopes to convince someone how terrible this is...