We investigate the veracity of the reports by Iwata & Umeno (2016, https://doi.org/10.1002/2016JA023036) and Iwata & Umeno (2017, https://doi.org/10.1002/2017JA023921), both of which claimed that the observed perturbations in GNSS-based ionospheric total electron content (TEC) could serve as a “precursor” of large earthquakes based on correlation analysis. Iwata&Umeno (2016) defined the spatial correlation of the residuals between the observed and predicted TEC time series and reported that the values are significantly larger before large earthquakes than those observed during non-earthquake periods. Iwata&Umeno (2017) claimed that the preseismic ionospheric disturbances can be distinguished from other non-earthquake phenomena based on the small percentage of area where the correlation value exceeds the criterion. They also claimed that the low propagation velocity of the correlation peaks is also a pre-seismic characteristic. Here we test their claims using a larger dataset. As a result, these three characteristics they claimed to have captured as evidence of earthquake precursors are not significant being frequently observed during normal (non-earthquake) days, and therefore we can find no basis for claiming that they are precursors to the earthquakes.