Monsanto was one of a number of manufacturers that the DOD ordered 2,4,5-T from, and they warned the US Government about the dioxin issue literally decades before they stopped using it, and they tried to raise at least some awareness of the issue before many other manufacturers even knew about it.

From the article that you linked:

Additionally "Boehringer, which used the relatively safer low-temperature-process since 1957, in the same year warned the other producers of 2,4,5-TCP, which were using the high-temperature-process, pointing out the risk[vague] and providing suggestions how to avoid them."[28]

Internal memoranda revealed that Monsanto (a major manufacturer of 2,4,5-T) had informed the U.S. government in 1952 that its 2,4,5-T was contaminated.[29] In the manufacture of 2,4,5-T, accidental overheating of the reaction mixture easily causes the product to condense into the toxic self-condensation product TCDD. At the time, precautions were not taken against this unintended side reaction, which also caused the Seveso disaster in Italy in 1976.

The employment of 2,4,5-T by the military rapidly ended, according to the American Cancer Society, following the convincing results of a study in 1970 that found 2,4,5-T could cause birth defects in lab animals.[30]

So yeah, Monsanto warned the US government of the problem 10 years before the start of Operation Ranch Hand, and they were ignored. And I can't really fault a chemical company for manufacturing chemicals when the DOD orders them.