It is believable that London would score poorly, but mostly as a result of the move to disincentivise driving with measures to restrict car movements, e.g. Low Traffic Neighbourhoods, Congestion Charge, Ultra Low Emission Zone, reduced road capacity for bus lanes, cycle lanes, pedestrianisation, removal of car parking, etc... Most of London now has a blanket 20mph speed limit which is growing year-on-year as part of a target to eliminate serious injuries and deaths from vehicle accidents.
Another consideration with this study is that for a city like London, with an extensive public transport network (as is the case in Paris and New York whom rank 3rd and 5th), vehicle congestion is less of a concern due to a lower mode share, but it would be disproportionately more problematic for cities which are more car dependent.
Image sourced from TfL: https://content.tfl.gov.uk/london-di...-limit-map.pdf