Oh for sure there is a ton that is going on behind the scenes when you substitute `np` --> `jnp`. But as someone who worked on gravitational dynamics a decade ago, it's really incredible that so much work has been done to make running numerical calculations on a GPU so straightforward for the programmer.
Obviously if you want maximum performance you'll probably still have to roll up your sleeves and write CUDA yourself, but there are a lot of situations where you can still get most of the benefit of using a GPU and never have to worry yourself over how to write CUDA.
People think high-level libraries are magic fairy dust - "abstractions! abstractions! abstractions!". But there's no such thing as an abstraction in real life, only heuristics.
Obviously if you want maximum performance you'll probably still have to roll up your sleeves and write CUDA yourself, but there are a lot of situations where you can still get most of the benefit of using a GPU and never have to worry yourself over how to write CUDA.