There are some ways in which they differ, such as when toxic refers to an actual toxin which is the product of a living organism while poisonous refers to a poison which may be artificially created.
Just for the record, above is the only difference mentioned in the M-W link given previously. I'm not sure that would apply in an industrial context like engineering.
I wouldn't have been able to formulate any difference between the two, and I doubt most laypeople are aware of any difference in ordinary conversation, except that 'poisonous' is an everyday Anglo-Saxon word and 'toxic' is a Latinate word used more in science and medicine. (And 'noxious,' much less common than either, might have a literary or historic quality.)
In the context given in the original post, my guess would be that they intend to contrast 'toxic' as meaning potentially dangerous in larger quantities or over a period of time, like cancer-causing chemicals, with 'poisonous' as meaning immediately dangerous, causing immediate symptoms or illness.
But it's only a guess. Perhaps some of the medical or pharmaceutical translators, or indeed the engineers, will actually know.