Yeah, he's really doing this -- he's challenging Puerto Rico's recent declaration that the death toll from Hurricane Maria was 2,975 rather than 64:

3000 people did not die in the two hurricanes that hit Puerto Rico. When I left the Island, AFTER the storm had hit, they had anywhere from 6 to 18 deaths. As time went by it did not go up by much. Then, a long time later, they started to report really large numbers, like 3000...

— Donald J. Trump (@realDonaldTrump) September 13, 2018

.....This was done by the Democrats in order to make me look as bad as possible when I was successfully raising Billions of Dollars to help rebuild Puerto Rico. If a person died for any reason, like old age, just add them onto the list. Bad politics. I love Puerto Rico!

— Donald J. Trump (@realDonaldTrump) September 13, 2018

Of course this is completely wrong. The Atlantic's Vann Newkirk explained how the number was arrived at last month.

... researchers from the George Washington University Milken Institute School of Public Health released a series of studies on Hurricane Maria’s impact in Puerto Rico. The mortality study [was] completed at the official request of Puerto Rican Governor Ricardo Rosselló and administered in conjunction with the University of Puerto Rico.

... the GW researchers essentially modeled what the death rate in Puerto Rico would’ve looked like if Maria hadn’t hit, and then used that number from September to the following February as a baseline. The estimate of real-life deaths above that baseline are the “excess deaths” estimated throughout that period.

The study, Newkirk tells us, "used official death certificates from Puerto Rico’s health department" to arrive at its estimates.

“The official government estimate of 64 deaths from the hurricane is low primarily because the conventions used for causal attribution only allowed for classification of deaths attributable directly to the storm, e.g., those caused by structural collapse, flying debris, floods and drownings,” the report notes. “During our broader study, we found that many physicians were not oriented in the appropriate certification protocol.This translated into an inadequate indicator for monitoring mortality in the hurricane’s aftermath.”

... while the storm did create a major lag in filing death certificates, physicians did eventually file records thoroughly and consistently.

So the method used to arrive at the estimate was careful and reality-based. It doesn't resemble Trump's caricature at all.

But it's still an estimate. Researchers can't brandish precisely 2,975 death certificates and say, "Here are dead people who wouldn't have died if the hurricane hadn't hit" (although it's clear that they've identified a significant number of such people).

Right-wingers don't believe in statistics. Oh, sure, they like the unemployment and GDP numbers now, because they're good and Trump is president. But they don't believe good numbers (economic or otherwise) when Democrats are in charge and they don't believe bad numbers when Republicans are in charge. Also, they don't believe numbers that challenge firmly held opinions -- they think undocumented immigrants are criminal-minded, so don't even bother telling them that the numbers contradict their feelings.

Right-wingers believe in anecdotes -- they point to the individual deaths of Mollie Tibbetts and Kate Steinle as "proof" that the undocumented are dangerous. They like to keep their arguments on the level of individual outrages. Trump doesn't have a specific outrage here, so he makes one up ("If a person died for any reason, like old age, just add them onto the list").

Conscientious people applying appropriate methodology are always at risk of ridicule from right-wingers -- it's an extension of the right's anti-science, anti-expertise bias. Conservatives regularly suspect researchers and experts of bad faith and political bias -- see the climate change debate, or the smearing of the inspectors who correctly determined that Iraq lacked weapons of mass destruction in 2003.

Trump is wrong and the George Washington researchers are right -- but they're researchers, so of course they're an easy target for conservative know-nothings.