Magic, and literally nothing else
Like you said in the question, preformationism was a belief held by scientists in the 19th century. But that was a grotesquely mistaken belief, and it fact the sperm is the less important half of the zygote. The female egg contains not only a set of 23 genes, but it also contains all the other components of the cell, including very important organelles like the mitochondria. In other words, even if you somehow got a 23-gene sperm to act like a zygote, it would die instantly, even it an artificial womb, because it has no structures within it.
So you'd need to now restructure how human sperm works, and have it somehow be a capable cell, (which in it of itself is a herculean feat) except now is when you run into a second problem - lack of DNA. Now, you specified sperm, which contains 23 genes. If you said you were using 46-gene somatic cells, you could (theoretically) just use clones. But you said sperm, which means we're starting out with 23 genes, and you need 46 for a human. Trying to grow a human with 23 is no go, so you could try duplicating them, except now you've managed to get an offspring with a 1.00 inbreeding coefficient, which is bad. As in, your description of them being grotesque is too mild for this, and they would probably die really fast, unless you (the donor) had near-perfect genetics.
Also, as a side point, you need to come up with the energy for an army that large to grow to maturity, and you've made no mention of a cloning factory on a rain-soaked water world, so I'm not sure how that stage of the process will work, but it's not as big of an issue as the rest of this, so I'll move on.
Now, for the final step:
These sexless humanoids have grown to be grotesque representations of
humanity, being completely deformed in appearance. They contain
exaggerated features, such as elongated arms, overly-large lips, giant
hands, etc. Being unfully formed humans, they also lack a large degree
of organs that people normally have. This should make them far weaker
than humans and unable to survive for a significant degree of time.
Instead, they are stronger and more resilient than normal. However,
their physical forms are biologically immortal, and possess a
remarkably strong healing factor that makes them difficult to kill.
So, we've established why these things are grotesque - a 1.00 inbreed coefficient will do that. Lacking organs also makes sense given that 1.00 inbreed. But here's what doesn't make sense - the fact that they ignore that. You can't ignore lacking a 'large degree of organs'. Missing a heart will kill you. Missing lungs will kill you. Missing a liver will kill you. Missing kidneys will kill you. Then there's the fact that they are 'biologically immortal'. Only a very specific type of jellyfish is biologically immortal, but it does that by reverting to it's polyp stage . And reverting to a polyp stage isn't something a human is capable of, because we don't have one - at best, it may be able to spawn an identical zygote, and that's just cloning. And, lastly, a 'healing factor' that makes something difficult to kill isn't something that will exist in a creature that can't manage to get all of its internal organs working. And even if it does exist, it shouldn't work at all.
Ultimately, you're trying to take the blueprints for a human, burn half of them at random, then try to build a better human out of them. Which is like taking the blueprints for, say, the Burj Kalifa burn half of it, and then mirror the parts you haven't burnt, and decide to try building something twice as tall. It's just not possible from a biological perspective. Mild handwavium won't let you get it either. The only way to do this is by just calling the whole process magic, and not go into the details.