This is from the OKLAHOMA GEOLOGICAL SURVEY OBSERVATORY.
thanks, this is from the arms control association
"Verification, of course, can never be achieved with 100-percent confidence. There will always be some level of nuclear testing below which the United States will not be able to monitor with high confidence using seismic means alone. Discovery of very small tests below the seismic threshold would depend on unquantifiable information from human or signal intelligence or photo reconnaissance. Although, the possibility of such information provides a further deterrent to the clandestine testing of low-yield nuclear devices, our ability to enforce the objectives of the treaty is measured by the efficacy of the monitoring system."
"The network is expected to detect all seismic events anywhere in the world of magnitude 4 or larger on the Richter scale, and to locate those events within a 1,000-square-kilometer error ellipse, the maximum area permitted for an on-site inspection under the terms of the treaty."
So they are saying the new proposed technology will have limitations. So I think it is safe to assume the old technology had greater limitations. Which means seismic detection has limitations and we do not have a magic wand for detecting nuclear tests, independant of whether this claimed test happened or not.