How does hobby programming and pure math degree compare with CS degree?

Separate from knowing the syntax and semantics of a programming language (which you have to know), Programming in the real world is about code herding and coding support tools . Each environment has its mix of Code storage location eg folders on a filesystem to GitHub repositories. Support tools include Editor / IDEs , compilers/ interpreters, linters, and Continuous Integration systems.

Your hobby programming has selection from all of those areas and it will be different from the program ecosystem at a job. Elevating your selection will give better understanding of the process but not the experience of working in complex environment with others. The selection of ecosystem in a CS school may not be as complex as real world version but "should" be more elaborate than your home brew environment.

All of this to say "it depends". If your homebrew ecosystem is working with a Open source project contributing code through Git(hub/Lab) using the GCC tools chain and VSCode you could be better prepared than the CS students for the real world.

Programming is a skill you have to learn by doing. Most CS students learn it during their degrees and the same goes for many math students.

In the end the CS students likely has more experience, while the math student has a better foundation. For a job experience is almost always more important, but depending on the job mathematical experience might actually be the one more important.