I think it's great that people are aware of this and defend against it happening. But the reality is that if Trump is SO unpopular to lose by 5 million votes, he's in big trouble.
After all, "could" is a wide word that cuts a huge path. And therefore the article is a math hypothetical. And hypothetically, Trump could lose by 25 million and still be re-elected. That's profoundly unlikely, but it's also not likely he'd lose by 5 million and be re-elected. Just mathematically possible. And important to be aware of.
Furthermore, hypothetical math aside, keep in mind that this "5 million vote loss" scenario presupposes the Democratic candidate getting an even greater number of votes in states that he or should would win anyway -- like in California, New York and Illinois -- compared to what the party got in 2016. But for that to happen, it would seem likely that most of those new votes would not be Democratic ones (since most Democrats who voted in 2016 voted Democratic), but rather from Independents and disenfranchised Republicans. Yes, some Democratic votes did go to Trump in 2016, but it wasn't two million. And if you're picking up Independent votes and disenfranchised Republicans in various states, there is no reason to presume that that wouldn't happen across the board, most especially those three states (Wisconsin, Michigan and Pennsylvania) that Trump squeaked by in 2016 by a total of 70,000 which gave him the election. The short version is that for the Democratic candidate to win the popular vote by 5 million, he or she will most likely have to pick up votes that aren't Democratic to begin with, since those votes in 2016 almost all went Democratic already. Which would be a major problem nationally for Trump.
Significantly more problematic for a candidate, however, is to go into an election where you may lose the popular vote by 5 million. For that to happen, the bottom really has to have fallen out of your campaign, and all those squeaker-thin wins you had the first time as a result of disenfranchised voters who wanted change are *likely* to totally disappear and be landslides for your opponent.
"Could" Trump lose the popular vote by 5 million and still get re-elected? Absolutely. And Democrats have to be aware and fight against it. But is it likely? I think such a possibility of losing the popular vote by 5 million would be far more terrifying to Republicans, who not only would *likely* lose the presidency, but also the House again and Senate, in droves.