/dev/null is not a database. By this logic is a hard disk a database, is a CD a database. No. They are storage mediums. You could store a database on them, but they themselves are not a database.
Considering there is no way to read back data written to /dev/null it will not be useful for storing database data.
It's nerd humor. You're not supposed to find it funny, but nod along approvingly while noticing how awfully clever you are for noticing the attempt at being funny.
The problem with the joke, as I see it, is that it only works with a colloquial definition of ACID. If it worked with a formal definition, it would be funnier by having a twinge of insight.
It's as if the joke requires two steps away from reality: first to apply the definition to a domain where it doesn't apply, then mis-reading the definition to make it fit. Having to go two steps like that spoils the feeling of insight that makes a joke like this work.
The article doesn't belabor the gag, which is a point in its favor. It has to hit you with its punch line and then quit before it wears out its welcome.
Considering there is no way to read back data written to /dev/null it will not be useful for storing database data.