Earlier quoted context omitted.
When I asked a few years ago, the brew maintainer said "parallelizing at this scale would hurt github, source: I work for github"
Definitely sounds like one of those "I don't want to do it because it's work, but I don't want to admit that so here's a spurious technical reason" excuses to me. If spikes at midnight etc are an issue just automatically disabled parallel downloads around midnight. Or only use them when running in a terminal. I really doubt it is an issue for GitHub though.
(It’s not just midnight, or terminals, as mentioned, not that either of these is really a “just”. And it’s not just GitHub, as mentioned.)