>Some of the points you raise are matters of style (I have files that are 10k lines long and think they are better that way
It's true that line counts are a matter of style. However, some coding styles tax cognitive bandwidth and immediate memory capacity more than others. This causes people to skim over specifics and miss bugs. Even if you are comfortable with a 10K LOC file you wrote, whomever reviews your code will probably be overwhelmed until they fully comprehend its structure. Grouping related things into smaller files is a way to focus their attention and communicate intended relations between code units.
> also I don't think unit tests are as useful as claimed
In general, I agree. Not a fan of TDD zealotry. But in this particular case some unit tests would be beneficial. There is a lot of stuff going in some methods.
They do seem to run PVS-Studio static analysis, which is somewhat reassuring, but I don't think that's enough in code that is so important, complex and widely used.