Fun fact: "/dev/nul" (with only one L) would have worked, even if there is no directory with that name.

That's been a feature since DOS 2.0, there was even an undocumented option AVAILDEV to make the prefix mandatory, instead of having device names present everywhere. But it broke the common trick used to detect if a directory exists ("if exist c:\some\path\nul").

Unix programmer remembered that in there's no /dev/null in DOS and that it's something shorter, and tried null which worked. Didn't check the directory contents afterwards. So basically your first sentence - doesn't seem at all unlikely to me. (I mean, I think it happened to me at least once too)

I've already created a 'NULL' file, but it was not a Unix thing... It was just because I got confused if it was NULL as in the programming languages I usually use.