We investigate the veracity of the reports by Iwata & Umeno (2016, doi:10.1002/2016JA023036) and Iwata & Umeno (2017, doi: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 a spatial correlation of residuals between the observed and predicted TEC time series. They reported that the correlation value is significantly larger before large earthquakes than those observed during non-earthquake periods. Iwata & Umeno (2017), who applied the same method to other large earthquake, 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 tested 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. In addition to that, the criteria of Iwata & Umeno (2017) cannot be applied to the large earthquake discussed by Iwata & Umeno (2016), and vice versa. Therefore, we can find no basis for claiming that they detected precursors to the earthquakes. The calculation procedure of the correlation function shows that the value is more of an indicator that amplifies small variations synchronized between nearby stations, like medium-scale traveling ionospheric disturbances rather than earthquake precursors.