The article implies that rockets burn 100% of their fuel, which is an oversimplification for most thrusters. There's also a fair amount of residual fuel and oxidizer in the piping, at least part of which is dumped into the environment. While it isn't that scary for something like kerolox, it's a concern for hypergolics.
Actually, this is mentioned in the article, in the "HOW MUCH DO DIFFERENT ROCKETS EMIT?" section, and gets more into it in the linked raptor article[1]
> Although all rocket engines do run fuel rich for the right balance of heat management and performance, so there is likely going to be unburnt fuel expelled regardless of the cycle type, but much more when it is an open cycle engine.
