It's an industry where many firms have had to develop their own network transport layers, database engines, languages, etc. Where machine learning and big data have been standard practice for over a decade. Where understanding complexity and concurrency theory are important, and designing lock-free algorithms can be a daily activity. There's very few areas of core CS that aren't used in investment banks.
Many firms have a strict policy of disallowing any source (open or not) from leaving company premises. Email servers and proxy filters have detectors that will raise an alert if source code is emailed or sent out over HTTP. It's easily a sackable offence and this discourages developers from contributing changes back to open-source: there are simply too many hurdles to jump through in order to do it.
But the real influence of banking is driving the development of stuff like RDMA over Infiniband, which eventually makes its way into other industries. Everything that "web companies" know (and take for granted) about scalability, performance, resilience, security etc etc comes from either investment banking or pr0n.
But obviously a lot of it remains behind closed doors, but there certainly some of that sort of thing going on.
Some of it also happens indirectly, for example a bank might design an improvement to the Linux kernel but because they want to remain covered by their support contract with say RedHat they might supply to the patch to RedHat and RedHat will then verify it and merge it into their core release and submit it back upstream, but it would have RedHat's name on it rather than the originating bank who might want to remain anonymous.
In most large finance companies though, it's also true that a lot of the less-exciting back office development work is being done by software developers who are overqualified for the roles but attracted by high wages.