That's why I'm not counting everyone, just the people that voted.

Winning the popular vote just means winning a plurality of the votes. He got more votes than Harris. Once you account for all the other candidates, though, his total comes out to less than 50% of all the people that voted. No one got a majority.

And in many countries if no one gets a majority there is a runoff. This is another structural problem that the US has.

The point being made is that in a healthy democracy, voting is either very high turnout or mandatory, as in some countries. It's also worth pointing out that getting 20% of all possible voters is an extremely weak mandate, and one of the norms we've been relying on is the idea that you don't have the right to fundamentally reshape the country according to your preferences.