Which repo were they referring to that was growing too big vertically?
Given its prominence in some of these other discussions from GitHub of a repo always on their performance radar now my gut instinct is that it could be the Windows monorepo.
Or alternatively someone is doing something very silly.
But there's a number of other big private repos with a high rate of merge on GitHub, and indeed it's currently a bit painful, e.g. the subroutine that check whether a PR can be merged tend to take ages. So my read is that they want to better support these big customers out of concern that they might leave too.
Every day, he sends hundreds of commits to his (most likely vibecoded) repositories of retro games recompilation.