4.18 was the second half of 2018, around the time linus took some time away and went off doing therapy to work on his “communication issues”.
I can't reproduce the exact datapoints from the site using `git grep`, but most of it seems to be down to a single commit that removed repeated usage of fuck from one file: https://github.com/torvalds/linux/commit/a44d924c81d43ddffc9...
(that was actually interesting to scan through, dunno why but i found the `[it] is fucking fucked` -> `[it] is broken` lines hilarious)