This is an interesting study but it does have limitations. Several are pointed out by the researchers in the research paper.

I think the most important limitation is that the results tell us that collision rates with pedestrians are higher with electric or hybrid cars than with cars with internal combustion engines, but the study can't tell us why the rates are higher. The researchers suggest that it could be because pedestrians can't hear the electric cars well, in urban settings. That's certainly a plausible possibility.

But there are other possibilities. If electric and hybrid cars are, on average, driven differently than other cars, perhaps because they are used for different purposes or have different types of driver, then that could be a reason for the different collision rates, rather than the actual type of propulsion or the sound it makes. The researchers mention that drivers of electric and hybrid cars tend to be younger, and that younger, inexperienced drivers tend to have more collisions, but that's not the only possible reason that's not directly to do with the car's engine type. This type of research can't distinguish between all these possibilities, and that's why further research would be needed.

Another possible issue is that the data used is quite old now, from 2013-17. The researchers point out that it's the latest available, but perhaps pedestrians and drivers have changed their behaviour as electric cars have become more common on the roads.