Picking up where we left off.
As everyone knew, Genestealers devoured their targets' flesh, analyzed the genes within it, then evolved organs suited to the local environment.
So what happened when they ate Astartes flesh?
A group of Genestealers had rammed their Space Hulk into a Strike Cruiser belonging to an Iron Hands successor Chapter. Flesh and machinery clashed, and the Genestealers ultimately won a pyrrhic victory.
The strongest surviving Purestrain Genestealer ate what little flesh the Sons of Iron Hands possessed. At that point, its desire for mechanization was not particularly pronounced. As everyone knew, Ferrus Manus had never wanted his sons to become "iron-handed" like him.
That was merely something a few fanatical worshippers of their Primarch had imitated, eventually turning it into a tradition.
It wandered aboard the ship as the vessel entered realspace.
The planet they reached next was the real reason they became so powerful.
Teks 23 World was an energy-producing world, rich in combustible gases and mineral deposits.
Naturally, it became a partner world of the Adeptus Mechanicus, extracting fuel and minerals, using its thermal energy for preliminary forging, and processing products to sell to the Mechanicus.
When this band of Genestealers arrived, they completed their final transformation.
In layman's terms, they became Iron Hands-Salamanders-Raven Guard hybrids.
They used the abundant gas and mineral resources to manufacture vast quantities of Jump Packs and Flamers.
The environment outside was harsh. Only a few harmless little creatures could survive there.
After eating those strange native creatures, the Flesh Change stemming from the Sons of Iron Hands' flesh inside them was also triggered.
They began mechanizing themselves to survive better outside.
This was not particularly serious. It even reduced their overall combat effectiveness, because mechanization weakened their speed advantage, and close combat was the Genestealers' greatest strength.