The ticket is suggesting that for building the BIOS that runs inside the VirtualBox image, they switch from OpenWatcom (another C compiler for DOS) to Digital Mars. VirtualBox emulates an IBM-compatible PC, and the traditional boot protocol for IBM-compatible PCs involves a BIOS that sets up the machine and provides a rudimentary runtime library for things like screen output or disk access. This BIOS is written for 16-bit x86, same as / binary-compatible with DOS, so any good compiler that can produce DOS object files can be made to work here too. (Most bootloaders / kernels will quickly switch to 32- or 64-bit mode, but they expect to be started in 16-bit mode and expect to make calls to the BIOS to load their own code.)

This ticket is not about changing the compiler that builds VirtualBox itself.