For a very simple reason. if Denva descends into eugenics, then it doesn't need genetic engineering to do so. Every historical expression of eugenics, every caste system, every instance of discrimination precedes genetic engineering.
If Denva descends into eugenics and we have given them gene-engineering tech, then they might use that to sterilize the undesirables. If they descend into eugenics and we didn't give them the technology, they'll just use chemicals, or mandatory anti-conception, or regular bullets. The effect is the same, the technology is irrelevant
I don't understand the issue with uplifting all of humanity, removing human defects with technology, and improving on the human baseline in all senses of the word, in longevity, in intelligence, and so forth. These are
all positive things that will help them develop and survive in an extremely hostile universe, wherein the slightest most precious of edges
must be applied, and holding the entire species back 'because
could be bad in the worst case' seems foolish. Even without it, large-scale high-level genetics is absolutely crucial to have any tool to detect and deal with genestealer cults, and given that we're next to a Tyranid splinter fleet, that is a real risk that they'll have to deal with.
Really, we're trusting them with most of our tech base, if you have so little faith in them as a society to not suddenly go megahitler, why the fuck are we even giving them
anything? Even the automation and manufacturing tech can be used to make an army of robots to enforce a regime's will. Juvenant
can be used to make a Cauldron-esque eternal shadowy caste system (but in this case, may be for their benefit). And so forth.
Fundamentally, whether or not Denva, in the long run, becomes a technocratic semi-heirarchal empire, a Star Trek-esque democratic Federation, or whatever else is ultimately up to
their society, so long as they don't become the Imperium v2. Neither democracy nor capitalism have exactly proved themselves to be the best prevailing ideologies, so I'm not married to them (the latter has been responsible for more death, destruction, slavery, ecological devestation, corruption, and so forth than communism
ever could, and the former mutated into an oligarchy near globally in a relatively short period of time).
Really, keeping them ignorant 'for their own good' is foolish in a immensely hostile universe.