I'm pretty sure it was a joke. It just wasn't that funny. Probably a good idea to clarify it though. For anyone that actually has no idea.
Generally the *.d directories. Are for drop in config files, or drop in files of some kind. One of their benefits is you can have the normal /etc file unmodified, and override configuration values in *.d files. If that program recognizes those at least. Then on updates you don't need to worry about issues, with the file getting overwritten or some kind of conflict. Since it won't touch the files in the *.d directory. sudo is a good example. You can make a sudoers.d directory. And put only the changes you want to make into a file in that. Most things use that basic idea for the .d dir.
Then you have things like the cron.d, modprobed.d sysctl.d That are sort of doing the same thing. But around their intended function. Like setting parameters, the modules that will be loaded, or specifing cron jobs.
Of course there are a ton of thing with .d. But they are almost always use in that way. Drop in configuration files with snippets to override defaults, or change some setting. Or a directory for drop in scripts of some kind, like init.d, and cron.d.
