Lean websites are NOT featured in the Top Stories carousel, only AMP websites, so this is not primarily about page speed. If page speed would be the main reason for AMP, they'd simply continue to demote websites that have poor performance in search results, and they'd curtail the main bloat from sites in Chrome, ads and trackers.

But I agree that Google has used this talking point successfully so far to bully publishers into giving up autonomy on their websites in exchange for being featured in top results, even when these sites have a great score on PageSpeed Insights.