The DL MM program is extremely generous, so I can't say I am surprised.
I switched to DL in mid-2019 and have flown about 80k BIS since then across all DL earning partners. What surprised me was to learn that I am already 25% of the way to earning 1MM. I spent years with Star Alliance before that doing 80k or more per year and my MM balance on United is less than that!
At that point I finally understood why the DL MM benefits were next to worthless. I suspect MM earning on DL will be far slower in the future with the program changes.
And then of course there are the people who have actually spent multiple decades on planes.