Those engineers are probably not working on AI. The traders were receiving phone orders from clients and executed them. Now there's a platform where clients put

Those engineers are probably not working on AI. The traders were receiving phone orders from clients and executed them. Now there's a platform where clients put in trades which get executed. The engineers develop the platform and enable the remaining traders to trade orders efficiently. It's probably at least 30% front-end web developers, many testers, some back-end developers. Execution only trading engines aren't that hard to develop.