Whilst it is abhorrent that Churchill tried to weasel out of paying compensation in that case, I'm not sure that article holds a lot of water.
Firstly, most trivially, regarding the studies they describe, if every one of those outfits was treated the same by drivers, that somewhat undermines the author's previous findings that "drivers leave less space when overtaking cyclists wearing helmets", as some of those outfits have helmets and others don't. Possibly this is controlled for in the actual study, possibly it isn't. As they haven't referenced the full study, assuming it is published, it's hard to know without doing further digging.
Secondly, their study didn't seem to look at how likely those cyclists were to be involved in actual accidents, only how drivers treated them when they did see them. Presumably this is a consequence of their trial design - without seeing the full, published data, it's all a bit pointless. We don't even know the conditions they conducted their study in - in broad daylight hi vis will affect a driver's responses less than in poor visibility.
But it's pretty meaningless - drivers who saw them weren't very considerate. Nobody really thinks hi vis makes drivers more likely to give you space - they think it makes drivers more likely to see you and not hit you, even if in not hitting you they only give you 3 inches of space. To claim that hi vis has no effect, you'd need to collate data from actual accidents and work out how many were wearing hi vis, how many weren't, and, from there, work out whether hi vis has any effect on the relative risk of actually being hit by a car. Maybe someone's done this - if so, it would be more compelling evidence than the data referred to in that article.