They’re not talking about a presidential election. They’re talking about the basic function of the federal government.
The House of Representatives and the President are both elected by the population. The electoral college fucks up the presidential election, and gerrymandering fucks up House Representative elections. But to spite that, both of them are voted for by the “majority”, and therefore they more accurately represent the people.
The Senate is, by design, not meant to do that. It is meant to give an equal voice to each individual state, regardless of population. Majority does not rule when it comes to the Senate. Again, by design. It’s a deliberately anti-democratic body.
They’re asking what happens when populations move shift around and congregate in a few states. This would result in a deadlock situation where the House and the President both represent the majority of people but the Senate would increasingly strangle their will. It would create a permanent minority rule over the country, and being Washington to a deadlock for…well until something is done about it.