But if you try to download in significant volume, their systems know it's automated and probably have signatures specifically for yt-dlp.

I think it's a bit of an escape hatch: they know they can't close the hole (after all, there is always the "analogue hole", or now AI doing browser control); and probably prefer leaving a small monitored and rate-limited door than trying to patch everything.

See: Google changing their sponsored link DOM/CSS/classes matchers every (few weeks - few months) to break ad blocker lists, but they're certainly capable/smart enough to do it daily or hourly, etc.