Why? Pagination seems like a good idea for really long threads.
Additionally, request count matters more than data transferred. It's much easier to serve 1MB to each of 100 users than 1KB to each of 100K users. n people are already going to view the comment thread for a submission - a several-dozen-KB increase in the amount of data that you send each of them (assuming you're serving them statically) results in anywhere from "little" to "imperceptibly" additional CPU load.
I'm assuming that what he means is that they'll replace the need to click on a link with the 'infinite scrolling' that you see on Fb, etc.