Their methodology was to compare the number of deaths per thousand reported by the Gaza Ministry of Health with the number of deaths per thousand of UNRWA employees, which were counted independently by UNRWA and not the Hamas ministry.
They found that the two death rates were roughly the same (actually, UNRWA's was higher), indicating that the health ministry was not exaggerating the total number of deaths.
The methodology is flawed. It makes an assumption that everyone in Gaza has an equal chance of being killed, meaning that Israel is killing people randomly and indiscriminately. This is not only a false assumption, but an antisemitic one.
Indeed, one reason UNRWA workers could have been killed at a higher rate is because none of them are children - and more than half of them are males of fighting age. About two thirds of the first 101 killed were men, a higher percentage than the percentage of male employees at UNRWA (about 54% across UNRWA, not sure about Gaza itself.) Chances are that at least some of the UNRWA casualties were also legal combatants, either actively participating or acting as "spotters" or other support.
But there's another problem with this. If the Lancet methodology is correct, then it should be correct for every month of the war, not just the first month.
But over the past month, from December 15 to January 15, Hamas claims 5200 people were killed - 2.4 per thousand. But only 15 UNRWA employees were killed in that same timeframe - only 1.1 per thousand! (I'm assuming 13,500 UNRWA employees and 2.2 million total population of Gaza, which seems to be The Lancet's numbers.)
Using the Lancet's own methodology, even assuming the false assumption that UNRWA employees represent the entire population of Gaza rather than only adults and primarily males, the number of deaths claimed by Hamas is over double what their methodology would predict!
That's thousands of false Gaza deaths - using the methodology of this peer-reviewed paper.
Even if we accept that the Gaza health ministry had accurate information for the first month of the war, logic indicates that as Gaza infrastructure has deteriorated, the reliability of their figures has gone down as well. As we showed earlier, even the UN no longer parrots the Hamas claims that 70% of the deaths are women and children. There is no reason to believe the ministry of health figures over the past two months given that we know their "70% women and children" number is pure propaganda.
Which strongly indicates that Hamas is mandating how many deaths to claim, and the relationship with reality is increasingly tenuous as the war goes on and as Hamas sees its figures accepted as truth worldwide.
Beyond that, would the Lancet even consider publishing a paper using the exact same methodology for the time period of December 15-January 15 as I've shown here? I highly doubt it. Which shows how even "peer reviewed" papers can be biased, even when their numbers are accurate - the researchers and editors are looking only to publish papers that confirm their biases, and they won't even bother to look at anything that shows that disproves their biases.
But if any doctors and statisticians want to take this data and submit it to this formerly prestigious journal - data that thoroughly rebuts the first paper - it would be instructive to see whether they are honest enough to publish it.
(If you are wondering, the data for November 10-December 15 shows a 2.5/1000 death rate for UNRWA employees and 3.5/1000 for Hamas figures of Gaza' general population - also a significant difference, but not as striking as the most recent month.)