When people talk about whether a cable is "standard" or not, they are drawing distinctions between different types of cable. By your definition every cable is standard, or at least every cable that can connect to two models by the same manufacturer. That's a useless definition. People don't mean that, they mean something adopted by ISO/IEEE or similarly available for anyone to use with a lot of adopters.
No, a "standard" means something that's well-defined that multiple parties can implement. Something that only one manufacturer can use is not a standard because nobody else can do it. Lightning is a standard, other people can and do make lightning cables and accessories, it's just a proprietary standard and AIUI requires you to be part of the Made For iPhone program.
That's actually an interesting point. Is it true, though? AFAIK you have to actually be a member of the MFi program to see its terms (and the terms are under NDA), so I don't know what the exact restrictions are. I'm certainly not aware of any non-Apple devices with a lightning port on them, but I'm struggling to think of who would actually make such a device. The only reason I can think of for this is if you want to make a wireless device that charges over a lightning cable, but it's almost certainly a lot cheaper for such a device to just charge over USB instead of trying to charge over lightning anyway, and the only reason to want lightning is if you want your users to be able to charge your device with the same cable they use to charge their iPhone, but nobody seems to care about that sort of thing.
Which is to say, this may very well be true, but the lack of third-party devices with lightning ports can also easily be explained as just nobody wants to do that.
