I mean.. people contributing to FOSS generally program in what they know - i.e. I have some time to contribute, I'll spend 10 productive hours in C, because I know what I'm doing, vs. learning Rust only to spend 30 hours and not really getting anything done.
I contributed to a Tcl/Tk library that I was using at work that had a specific issue with some image files, so I fixed it internally, and contributed the fix back to the FOSS project (with permission from work).
People working at Microsoft in the SharePoint team also program in a language/framework they know (and they must be masochists if they're working with ASP.NET WebForms). Knowledge of the language doesn't prevent vulnerabilities.
I contributed to a Tcl/Tk library that I was using at work that had a specific issue with some image files, so I fixed it internally, and contributed the fix back to the FOSS project (with permission from work).