All you need is a client-side cache.
(It would perhaps be more accurate to say that React folks had to reinvent these patterns, but the problems were definitely present in the postback era)
It doesn't. How often do you have the same information redundantly displayed in multiple places, on the same screen?
The comments total at the top and each of the thread counts need to be kept in sync with their corresponding inline list representations.
Most lists in apps display a total count summary. This is even more common on desktop, where you have room to display more info.