For reasons best known to the CIAIC, the plots (and therefore very likely the speeds) appear to be based mostly on primary radar rather than FDR or ADS-B data, so a little wiggle room isn't altogether surprising.
Based on the timelines elsewhere in the report, both aircraft appear to have been travelling at around 142-143 kt at the point of minimum separation, which could indeed have been a bit (but probably not a lot) less than the quoted 520 m.