This is a good qualification, but not all "products" need be consumer-facing products. That's just a play on words for "retail" sales vs business or professional sales.
Its a seperate point to say its a 'feature' not a product and thus is not actually sold to anyone (and thus distinct).
It's a huge distinction if formal legal action happens because Apple can establish quite easily that they have relied on the Kit format for previous software, whereas HealthKit.com doesn't (AFAIK) have any major traction in the software space. Law student here, IANAL, so take this with a huge grain of salt.
Either way, in Australia, both parties would apply under Classes 9 and 44 which covers all computer software and medical services respectively. It's likely that Apple will win on class 9 as "Kit" is far better known in the software community than "HealthKit.com"; and arguably 44 too, depending on Healthkit.com's traction in the medical field.
It's unlikely that Apple's HealthKit and HealthKit's HealthKit would cause confusion in the market - but it makes a silly story to say that Apple's Health (which may have overlapping functionality with the preexisting HealthKit).
And regardless, there's unlikely to be overlap between the markets for these two products. HealthKit.com is somewhat hard to navigate, but the impression I get is that their product is aimed at medical practitioners. I doubt they overlap with the mobile developer demographic to any significant extent.
Though I'm curious to hear from someone with more legal knowledge than me: would the fact that there's very little chance of collision between these names protect either party from legal repercussions?
What about the effect of search engines on brand value? Whereas, before, the existing HealthKit was probably the top result, after an invasion of Apple HealthKit development sites they may be kicked to the third page hellban.
Naturally, this same reasoning can be used to challenge the use of the same trademark in a different domain, something clearly accepted by law.
The fact that has Apple has more money than they know what to do with is the real factor for legal repercussions.
It's one thing to have theoretically defensible IP, and other thing entirely to actually be able to.
Assuming equal resources backing the legal teams, it would probably be sufficient that they are both forms of software, as all software trademarks are lumped together in the single category. There might be some sort of passing off action one way or another but it would be more likely to come from Apple than HealthKit.
There seems to be overlap between the Health app and Healthkit.com which I suppose is the real issue. Hard to tell if Healthkit truly got sherlocked though, because I can't tell what they do from their website past management software.
Its a seperate point to say its a 'feature' not a product and thus is not actually sold to anyone (and thus distinct).