It's not 20 million. Furthermore, you're comparing 2020's final total with an election that is still being counted. California is at 63% reporting right now, they alone will eventually add millions more votes to the final overall total.
The final number will probably be around 7 million less votes than 2020. But add to that an increase of about 3 million registered voters.
So the point still holds by about 10 million missing votes.