What further business problems is he creating?
A company can't afford to pretty much be dependable on a single person, what happens if they get sick or decide to leave?
Being a star employee is good, but it always comes with the risk, people think that being indispensable is good, it might be good in a small startup but not in a large company.
At the end regardless how good they are they are only a small part of the delivery bandwidth of the company, and regardless of how good they are they can't do the job of 10 people even if they are smarter than 10 people combined.
As a developer their job is to produce reliable and maintainable code, this means that the code can be picked up by anyone in the company and worked with, if they don't they effectively create a bottleneck and a dependency on a human resource which is a huge risk for any business.
It's ok to be good, it's ok to even be better than some, or even the rest as long as you are a notch above the rest primarily due to soft skills, if the product you produce is effectively alien to the rest of your team and the company then what you produce is a liability not a solution.
There's also a couple of serious red flags in describing a contract as "In my contract, I had to work ALONE in order to build a complete software alone, with my own programming principles. I was recruited BECAUSE the team has no skills at all in the demanding fields."
Which means there's no use of code reviews, even as an opportunity to educate someone else. There appears to be no process for knowledge transfer at all, so how is the company going to gain these skills? Separate principles again suggests not writing code in the same way as everyone else.
Plus if you're concerned after 2 previous incidents of this happening to you and you find yourself months into a project being left on your own then some pre-emption might be useful. Write lots of comments in your code, talk colleagues through it, suggest training videos on the languages technology - make yourself useful and available.
A contractor that would produce some fully functional, and even elegant in it's own way but convoluted piece of code is doing a poor job regardless if it's because the code is so smart that almost no one can understand it or because it's utterly inconsistent internally and is an unreadable mess.
You aren't selling a binary product here, a company isn't buying off the shelf software, they are contracting you to further develop their IP if what you produce is not useable to them you effectively wasted their time and money.
So the problem is that the other people in the team are too lazy to learn to understand coding techniques that are a little less trivial?!
Whatever else his code is (and remember, we only have his opinion that it's good, we can't judge for ourselves), he's failing really badly to communicate with his co-workers and bosses.