Of course it's still deterministic. It's a computer program, so it has to be deterministic. Multiplayer still uses a lockstep simulation, so all clients must compute the same random outcome.

But it does introduce an element of randomness that isn't present anywhere else in the game. Every other recipe in the game has fixed inputs and outputs with known ratios, and often very nice ratios. Put in two iron plates and get out a gear. Put in one copper plate and get out two copper wires.

The randomness in uranium processing is used to simulate the cascades of centrifuges used for isotope enrichment without having to track the enrichment of every single lump of uranium, and without having to introduce a hundred different types of uranium ore items. Instead you have a 0.7% chance of getting a lump of uranium-235 every time you process some uranium ore; the rest of the time you get uranium-238. 235 is used for fuel, 238 is used for ammo and for further enrichment.