So you learn how the different pieces of the hardware work, how do you instruct them to do things, what are their limitations and so on. And once that clicks, it's not particulary more complex than writing modern code. Like programming today, it's less about typing and more about thinking about how all the dots connect together towards doing what you want to do. You do end up with a lot more code that does a lot less, since there are no abstractions and you do everything by hand. But otherwise it's something that anyone with aptitude for programming can pick up.
The fantasy console that Pico 8 presents is still way more powerful in most ways than an NES though.
32KB cartridges
32KB RAM
a 1MB RAM limitation for the lua VM