The main point is the uncertainties are underestimated, cause you can confuse a heavier Z boson for a W if one of the muons to which Z decays escape detection a

The main point is the uncertainties are underestimated, cause you can confuse a heavier Z boson for a W if one of the muons to which Z decays escape detection and so the event looks like a muon-neutrino decay of Z. The author seems to doubt that the way the data is corrected for this is really that precise.