It's not "we don't know", it's that there is "no established relationship" ...
Once again higher temperatures are not a cause of increase in other forms of extreme weather like floods, droughts, tornados or hurricanes ...
Even the data that the EPA uses from the NOAA disputes that ...
There is "no established relationship" is not proof that there is no relationship.
Absence of proof is not proof of absence.
But drawing the connection between warmer temperatures and more complex weather events like drought, flooding, hurricanes—or even the expansion of the Antarctic ice sheet—is way more difficult. “For events like that, it’s not clear that we have an established agreement in the scientific community,” says Stephanie Herring, a NOAA climate scientist and editor of the report. The problem is complexity. Or maybe the problems are complexities. Anyway, it’s confusing. The key metric in these models is called fraction of attributable risk. That’s a measurement of how much stronger, higher, swirlier the thing being tested for—rain, temperature, wind—was in the emissions-heavy model. “For example, in a world without greenhouse gases a heat wave might come in at 5 degrees above normal,” says Herring. “With climate change, you add an extra 1.5 degrees.” Flooding is only slightly more difficult to suss out than heat waves, because scientists have a pretty good idea of how heat and moisture interact in the atmosphere—but still, the models don’t always agree. In these studies, authors determined that several of the droughts they looked at had no discernible links to climate change. Prior to that, the sample size is limited to storms humans or their weather stations sat through first hand. “Because the hurricane data record is so short, there’s a lot of uncertainty,” says Cullen. But revealing those blind spots is one of the goals of these studies.
This article is moreso about some uncertainty in how climate affects extreme weather. There's a ton of uncertainty because these are rare events, and we likely can't link some droughts to climate change. Whether that's because we don't know enough, or because we need a better model, or whether it's because climate change absolutely 100% doesn't cause those things; we don't know the answer.
Likely is not the same as absolutely, especially when it comes to ridiculously complicated areas of study like climate.