What is "the Biden wall"?

I'm trying to read "Mexico’s coronavirus deaths are plummeting. The ‘Biden wall’ could be a factor" (WaPo) — just trying to understand what the headline means.

I'm glad to hear that the coronavirus situation in Mexico has improved, but why bring Biden into the picture and attempt to give him credit? He could be a factor. Because something he did is susceptible to being called a "wall." Can't we just be happy for Mexico and give credit to Mexico for competence? Why must it be about us? I have to presume that we are self-obsessed and partisan and WaPo is dedicated to making us click and then feeding us with pro-Democratic Party material.

Let's see if my presumption holds up. There's this in the 4th paragraph:

In addition [to things Mexico has done to control the virus], U.S. vaccinations appear to be blocking the southward spread of the virus.

Then, finally, in the 8th paragraph, we encounter the term the "Biden wall":

There may be another, intriguing factor. Malaquías López-Cervantes, a professor of public health at the National Autonomous University of Mexico, dubbed it the “Biden wall.” With nearly half the U.S. population vaccinated with at least one shot, he said, fewer infections are being carried to Mexico....

So... the "wall" is the way there are lots of people going back and forth between Mexico and the United States but because of vaccinations in the U.S., there is less virus going along with them. Quite aside from whether Biden deserves to have his name on the vaccination effort — why not Trump's?! — it's perfectly silly to call the continued flow of people between the 2 countries a "wall." 

I hope Professor López-Cervantes intended to make fun of Americans when he chose that hot word "wall" to hook us into his theory. I'm not doubting that the theory is correct — that less virus in the U.S. means less virus carried into Mexico — I'm just interested in his deployment of the term "Biden's wall" and the way The Washington Post snapped it up and propagated it. It's an idea-virus that made its way from Mexico to the U.S.