I beg to differ.
Right, a single book has on the order of 4-500 pages, longer ones have more, but we'll take the average, and an average book has....30 lines of text per page, so 13500 lines per book. That makes the kernel ~1000 books, which is a lot of books, but avid readers do read 50+ books per year (my father probably does close to double this). That makes reading the entire source absolutely possible, in the order of 10s of years, which is a long time, but then, the kernel has been around for what, 25 now? So there are a number of maintainers who have been around long enough to have read through the entire kernel.
Compare that to google, where[1] there are almost as many unique source files as the kernel has lines (though to be fair, many are autogenerated).
[1]: http://cacm.acm.org/magazines/2016/7/204032-why-google-store...
Also, to be clear, Linux is relatively small compared to Google or Microsoft, or indeed many corporations codebases. Someone could conceivably read the entire Linux kernel codebase. That's not true for BigCorp.