Well, I did a test:

All four physical cores are working in generating the accompaniment and playback of a song with BIAB.

I did load a busy song (many changes and five RealTracks), wait until the computer was as quiet as possible then hit Generate and Play to make sure it does generate something.

All four cores started to work to generate the accompaniment and kept working while playing the song.

Regardless on how the software development process works and what language is used, I do definetely doubt that the compilers to generate the dlls and exes are as ancient as some of the code might be. Modern compilers will probably generate parallel threads if not forced to generate one sequential running program.

Desktop; i7-2600k, 8 GB mem., Win 10 Pro, BIAB 2017; RB 2017 - latest build
Laptop: i5-2410M, 4 GB mem, Win 10 Pro, BIAB 2017; RB 2017 - latest build