This is an interesting result, but not a very surprising one. xorshift128 is a fast PRNG. It’s useful when you want a deterministic sequence for testing purposes, it’s not a cryptographically secure random number generator. There’s a simple arithmetic relationship between each number in the sequence, with a tiny amount of hidden state. This is exactly the kind of thing that a neural network can learn.
I would be very surprised if this worked on even something like a sequence of MD5 hashes of the outputs from xorshift128, which contains some of the basic building blocks for a cryptographically secure PRNG but definitely doesn’t come close to actually being one.