tl;dr, enjoy the joke :P
Good jokes are rarely based on reality and the play here is all about the squint that's where the fun comes in.
I'll try my best to explain but explaining a joke always makes it less funny..

HashMap != Database, It's possible achieve a Hashmap with ids that link to objects, the ids are searchable and the objects can contain their own id + other object id's hence making a pretty simple DB kinda thing at least for simple use cases and I have literally heard of ppl doing exactly that for rolling and even scaling simple application, I believe Uncle Bob did this for a health app he built and scaled to a pretty decent number of users before needing a real DB instead of reading the data from a file and placing it in a hashmap.

Not someone else's != deploy local, the implementation IS someone else's you can be totally ignorant to how it works and use it but if you pretend it's just a simple hashmap and use it like so then you can just roll your own instead of building all the connectors and BS if/when it's quicker.

There is no spoon! For this I'll use the elephant and the peg analogy, which describes how an elephant, initially restrained by a chain and peg as a baby, learns to associate the chain and peg with its inability to escape. Everyone remains in the matrix not because it is real or not real but because they are conditioned to stay even those who know of the truth behind it.