I think people are confusing exactly what 40k Admech is, they do not develop, create or EVEN tinker. All of that is heresy with the only forgeworld that has tinkered with anything being Stygies VIII and they are currently down the barrel of multiple Deathwatch investigations/purges.
The Admech of 40k would rather fall back on a 20th century hovercraft that was lost in the back of an STC than ever replace the destroyed Plamsa coil of a Grav tank that they no longer have the STC plans for. The hovercraft is perfectly inline for the story of 40k Admech, we do not have the beautiful high tech plans and vehicles of 30k where they still developed and did science. If you replaced something in a tank or even duck taped a shield generator to save a Hive world you would be burned alive as a heretic.
The last time I read of Admech creating something from nothing was from the The Beast Arises in early 30k when The Priest Dominus strapped his crashed ship's main Plasma Cannon to the destroyed but repaired tracks of a command carrier.
Were in 40k people, A spanner and Duck tape will get you shot and this is why there is no over arching theme to our army. We get what the STC says we get, no deviation, no tinkering here or there. If the only thing the database says it has is a rubber dingy and a grenade, we get a rubber dingy and a grenade.
I don't entirely agree with this as 40k fluff is never entirely consistent. For every story of someone being persecuted for tech heresy I can probably find one of someone getting away with it. The only thing they had been consistent about up to this point was the declining tech base.
Also STC tech was the height of human accomplishment, this is not that. It's dumb by our standards let alone theirs.