The think is, if you are on a internet connection where you have frequent loading time of 3s for the _basic_ content, then you are used to seeing partial loaded pages and you likely love it if the connection happen to load the content you care about first, potentially leaving the page before it's completely loaded when noticing that it's not what you are looking for.
The choice of 8s looks _a lot_ like they where looking for the highest time they can chooses without making it look obvious deliberately but which is long enough to make many "causal" users stop using the script blocker.
I.e. 10+s would look way to obvious as a intentionally long choice. 9s still looks big as we are used to see 9 as nearly 10 from product prices to some degree (e.g. 9.99€). So the highest number which doesn't look obviously high is the 8. ¯\_(ツ)_/¯
It only looks malicious because you're looking for it to be malicious.
Better yet, make AMP opt-in.
It’s naive to think AMP has anything to do with improving the UX, it makes it worse in every way.
I really only care about the text on most websites, but I end up waiting for a lot of assets before I can see that. I understand why google wants to avoid having the page redraw the layout as assets arrive, that can also be annoying if done with a lot of relative formatting.
But for me, on average, the experience is worse than non-AMP sites.
The CSS animation is a fallback to that intended behavior.