Interesting. That means they actually get all of the trader's source code as well. Unless there was a royalty involved, this isn't such a great deal IMHO.
I guess the thing to think about is whether you can make more money by yourself (catch what you kill) vs a secure ~$200,000 salary + % sizable performance bonus. The second route is also more of a career than the first's risky venture, meaning after say 5-7 years, your total compensation can exceed $500,000 and will only go up. These companies usually tend to have better infrastructure (faster execution and faster access to real-time data), quality backtester that is built in-house, and fee negotiations with exchanges, resulting in much less latency and transaction costs. Plus instead of competing with other traders, there is more collaboration. You might be skilled in machine learning and predictive analytics, but the trader next to you may have specialized in AI/deep learning, and the trader to your left may be skilled in market microstructure and low-latency development. I always find it puzzling why people go for companies that forces you to you use your own capital and provide no salary. If you truly are that talented and can earn $500,000+ a year trading, then you might as well start a hedge fund/asset management firm.
There happens to be another person from ET who asks me about vertical DOMs and scalping with Sterling. Maybe I could provide you some useful info. It seems not that easy to get what you are looking for.