Your math is clearly wrong as it isn't 21 million...
When this stupid argument started, there were still a ton of votes being counted, even if they knew whom won the state, they didn't know the exact number of popular votes.
As you can see in the graph below, when this started, even Trump had less votes in 2024 than he did in 2020.
But that's not correct, as they keep counting, Trump got more votes in 2024 than he did in 2020.
2020
81.28m for Biden
74.22m for Trump
2024 (so far, and they might not be fully done counting yet)
74.25m for Trump
70.35 for Harris
So instead of your 21 million, it's actually less than 11 million.
And really, one would need to go into more details of which states (or if it's about equality among all) the that voted less, as a lot of states are basically known which way the state is going. So less people might show up.
It's really only the battle ground states that matter big time.