After poking around the site, I can't see any specific reference—which makes me suspect it was an independent discovery of a cool idea—but this is very similar to a deliberately-constrained version of the Join Calculus https://www.microsoft.com/en-us/research/wp-content/uploads/... which uses a similar chemical-ish model of computation. There's an implementation of the Join Calculus in an OCaml-like language called (of course) JoCaml. Where Alchemist would write
2H + O -> H2O
JoCaml would write
def h() & h() & o() = h2o();;
One of the big differences is that JoCaml allows you to embed payloads inside your atoms, e.g.
def a(x) & b(b) = c(if b then x else 0);;
will consume an a with an integer payload and a b with a boolean payload, and produce a c with an integer payload. It also lets you include computations as reactions happen, e.g.
Yeah, Stochastic Pi-calculus and chemical reaction networks simulators are doing this this too. They actually precisely define the probabilities of reactions based on the current amount of different molecules and their reaction rates. For instance, I used the tool called Beta Workbench (BetaWB) and hand-written simulators for modeling systems like this. It's a really cool computational model, I think.
Adding relative weights to each rule would be a simple extension to the grammar. If only integer weights were permitted it could be a simple preprocessor.
There is a very detailed tutorial (which appears to have sadly half-broken formatting) which explains JoCaml in more detail here: https://sites.google.com/site/winitzki/tutorial-on-join-calc...