A typical true believer's mischaracterization of what Klass was saying.
Klass' belief was that unexplained aerial phenomena ultimately would be proven to have prosaic explanations, even if they could not be resolved at the time, whether by lack of data or incomplete gathering of evidence. His point was that too many of the True Believers were ready to jump to the ET explanation, even though that was the most unlikely of causes.
One aspect of UFOlogy unexplored by most researchers is the possiblity of undiscovered natural explanations for UFOs. Hyaek was widely ridiculed for his "swamp gas" explanation back in the 1960's, but the fact remains that there are many natural phenomena on Earth that are unknown and/or understudied. An exmaple might be natural piezoelectricity -- build up of large static electric charges in quartz-rich rocks in the Earth's crust, suddenly discharged by the strain-relief of a major earthquake. Such discharges could ionize gas and produce flying, glowing "spheres" moving at high velocity.
The problem for UFO skeptics is that while many of their natural phenomena explanations are reasonable and plausible, on the particulars, those explanations are often conjectural and inconclusive and sometimes rely on what is otherwise regarded as fringe science. The UFO subject remains fascinating in no small part because the better advocates and skeptics are all pushing against the mainstream in taking up the subject, attentive to the evidence and to scientific and technical issues, and careful in their arguments. Many UFO skeptics are believers in the possibility of UFOs as ET visitors, and many UFO researchers are fierce debunkers of much supposed UFO evidence.