After we know who is "they," or what "they" are doing, we can offer an opinion.

hornworms, caterpillers etc. Little cute wormy things.

Dipel (Bacillus thuringiensis-kurstaki) is very helpful in controlling caterpillars, as long as they (the caterpillars) are very young.

bt is pretty safe and effective.
I imagine if you're in the USA, it's pretty hot right now...
brassicas are generally a cool-season crop. Are they planted for autumn/winter?
Whenever I've planted brassicas out of season, they've been stripped by white butterfly caterpillars.
It also takes a while for parasitic wasps to get going and killing off all their dinner has its own set of consequences!

I can find nothing that says Diamotaceous Earth is appropriate for Tomato Hornworms, Cabbage Loopers, Imported Cabbage Worms, etc. so the wrong material was used to try to control these.
Bacillus thuringiensis - Kurstaki, which Dipel is, could be effective when used at the proper time. Insect control starts with using the right methods at the right time. It is a learning process that takes some time.

The diatomaceous? earth wouldn't help with a catepillar problem, at least not in a timely manner. Typically you use diatomaceous earth for critters that spend much of their time crawling around on the ground. I've never heard of local gardeners employing diatomaceous earth in their veggie gardens, though it might be helpful if you have a slug prolbem in your garden.

I have used bacillus thuringiensis (aka Dipel dust, Thuricide) for many years. It works wonders in controlling all sorts of chewing critters. The plants seem to be oblivious to the dust, meaning I've never experienced plant toxicity related to an overdose. It doesn't affect bees (bees don't chew on your plants). And since the stuff is wholly non-toxic to warm blooded animals, you don't have to concern yourself with hurting your pets or children or any residues that manage to make their way onto your dining room table.

Me, your information regarding Dipel and Thuricide is not 'quite' accurate. It doesn't control all sorts of chewing pests....caterpillars ONLY. And not chewing larvae that sort of look like caterpillars, such as sawfly larvae. Only the larvae of moths and butterflies, skippers, hairstreaks and all of their cousins are susceptible to this very specific form of biological control.

Again, Bt is effective when these larvae are young, so it pays to keep inspecting your plants for eggs and tiny hatchlings. Lepidopteran eggs are about the size of the head of a pin. Thus, the baby caterpillars are pretty darned small, but we rarely think to look for them at that stage.