So there are three main possibilities.

1) Human gene experiments.
2) Archotech civ experiments.
3) Evolved on their own.

The tribal and plant-loving nature of thrumboian suggests some kind of predisposition for more agrarian societies at the expense of advancement. Rather than having the cultural potential for advanced technology by reverse engineering more intellectually stimulating machinery, they seem to be perpetually stuck in the pre-industrial revolution era.

I guess a few questions... are they on every planet, or just a few? I would wager that they are not on every planet, if only because it would be somewhat difficult for them to desire to travel everywhere, and a humanoid intelligent being that has a great similarity with another creature that is very desirable for many reasons makes their own evolution too far-fetched.

Additionally, human gene experiments seem like they would be a lot less clean and give birth to a lot of "failures" with a more mixed bag of traits. Even if they did manage to successfully create a thrumboian, I doubt that a glitterworld capable of something like this would just serenely give them free will.

I suppose it is possible that an archotech did something to the thrumboians after they were created in order to neuter humanity, but affecting human brains at a distance with fine adjustments seems like something beyond even archotechs at the moment. (Psychic weapons and the lot seem to come with a bunch of potential drawbacks, even within the range of a single tile. Psychic soothes and drones only affect on a pretty minor level, so i think affecting something like the genetics of thrumbos might be beyond archotech ability at long range.)

So then, my conclusion is that they were created by some archotech for reasons only it would know. Seeding them on planets would be easy, and if they're not on every planet, this would be a reasonable suggestion. It would also remove the human cruelty element from the equation, so genetic disposition to obey humans and such things would not come into play, as you might expect from how they act.