No, they argued that the best fit of reported results does not match the best fit of other pandemics. A linear fit would not correlate as well, and an exponential fit does not correlate as well, either (and an exponential fit is the expected fit based on epidemiology). China was reporting 10,000 infected while the Lancet was estimating 75,000 so skepticism about the numbers is hardly new.
A death rate under 2% is unimportant if the rate of infection is low and the impacted population is small. As of two weeks ago, 35 million people in China were under quarantine. Assume that eventually, 10% of the quarantined population becomes infected. Seventy thousand people will die that wouldn't have otherwise. Ordinary flu has, for the past several years, had a mortality rate of 0.05%.
So far, the new coronavirus, dubbed 2019-nCoV, has led to more than 20,000 illnesses and 427 deaths in China, as well as more than 200 illnesses and two deaths outside of mainland China. But that's nothing compared with the flu, also called influenza. In the U.S. alone, the flu has already caused an estimated 19 million illnesses, 180,000 hospitalizations and 10,000 deaths this season, according to the Centers for Disease Control and Prevention (CDC).
Presume COVID-19 is ten percent more infectious than garden-variety flu and ten percent more lethal. That's still raggedy-bad. Presume instead that the current estimates (R0 2.2 compared to flu's R0 1.3 and 2% mortality compared to flu's 0.05%). That is beyond raggedy-bad. No doubt: it ain't airborne rabies or weaponized ebola but the whole point of the argument is that there is ample reason to believe China is under-reporting.