If the source file size matters you're really not using your editor right. With a modern editor it shouldn't really matter where something is, you'll always just directly jump to the symbols.
And that github source view isn't really that great anyways. For any serious reading better clone it. Apparently the billions of VC capital weren't able to buy a proper source code xref setup where you can actually jump to symbols directly, which is like the most important feature for any code review.
I guess the maintainer was just not very interested in trivial submissions. Yes splitting up source files is trivial.