There's obviously a big demand for this so either you're wrong or people using it are actually programmers.
The target market is those people that sit in places like the AWS loft or places like that, listen to tech talks and wish they had a coder to build their ideas for free.
Most of them already know what an API is. Even if you don't use the external data sources, what bubble provides is somewhat enough to win hackathons and get funding provided you can present and sell.
I've seen a bunch of traders, business people and otherwise people with random ideas that fit in that group. So yes, not at all surprised they make that much money.
Ah... now, in my mind, you're getting into Software Engineer territory. Programmer, to me, simply means you know how to make a computer do what you want. That can be "knowing how to search for someone else's code that makes the computer do what you want" at times. Once you get into thinking about the problem and how that related to what computers do, you're getting out of that generic "programmer" term and into engineering.
If you are a pro, you would know the industry, the hardware, how things communicate, how and why to scale and report things at a higher level, because you've been exposed to the needs and wants of an Enterprise.
Kind of wouldn't surprise me if the user numbers had a decent sized (probably not majority) programmer contingent; these kinds of services are a good way to get a quick prototype out, before you legit build something.