I'm not surprised it's possible to beat algorithmic detection and of course it's an arms race that will go on. I don't really belive in the concept of turnitin etc and if you treat people so impersonally it shouldn't be a surprise that they look for ways to get around it.
In context, it's usually very easy to read something (I'm thinking of students writing) and know if they wrote it or not.
Happy for your success though. You saw demand in the marketplace and you capitalized on the opportunity! Wish I had the same initiative.