Bike lanes (or, rather, just generally improved attitudes to municipal biking infrastructure) in cities that have adopted bike share schemes seems the most likely cause if safety gains in your first quote.
Helmet-wearing cyclists being more likely to cycle in unsafe areas seems far less plausible. Do you really believe this is the case. At least anecdotally, I would say the opposite.
I don't necessarily agree with op, but you rarely see cyclists with helmets in amsterdam and other dutch cities where a lot of people use cycling as their default transportation method, and where as a cyclist you have a priority over car drivers, whereas in places like berlin you have much more cyclists wearing helmets, and rightly so. Though I have to say that I myself only wore the helmet while working as a bike messenger, and rarely while not. So obviously on a day where I'll be biking for 6-8 hours I'm more likely to get into an accident, helmet or not.
Ah, ok, I'm starting to see a different perspective on helmet use: comparing cities rather than individuals in a city.
Perhaps the studies should differentiate these: i.e. comparing accident rates of helmets against non-helmets within each city individually, vs comparing e.g. non-helmet-users in Amsterdam against helmet-users in Berlin.
I live in Dublin which, as mentioned in the article, has recently seen marked improvements in cycle safety. Helmet usage here is high but under 50% and seems evenly distributed. I'd be curious to see the stats on what the means for accident rates local to here.
I only wear a helmet if I am going to be biking along a busy road, and I bike more dangerously with a helmet because there is less risk of getting hurt in a crash. So it makes sense to me.
Same as driving a "safer" car. Cars are MUUUUUUUUUUUUCH safer than they were 20 years ago. Yet car injuries are not declining. You'd think we would be waaaaaaaay safer.
Driving and biking is a large mental effort. The brain does this because it knows it will die otherwise. The safer the environment the less the brain cares about putting full power into attention to the activity.
DHH Mentioned this about F1. F1 is WAAAAAAY safer than it used to be, yet more accidents. Why? Because when before people realized that speeding down a sharp turn could lead to a fatal cliff fall, now with the guard rails they can push themselves harder without as much risk, so they do, and end up crashing.
That depends a lot on location. Biking at 15-20 km/hour on segregated cycle paths that you know well, in a society that knows how cyclists behave, is very relaxing.
Helmet-wearing cyclists being more likely to cycle in unsafe areas seems far less plausible. Do you really believe this is the case. At least anecdotally, I would say the opposite.