Join Date: Sep 2010
Location: In the shadows ...
They could always have just come across some of these jetbikes? In a galaxy of a million worlds with a thousand chapters and untold amount of secret caches, vaults, or other things. It wouldn't be bad fluff to say they came across them but obviously lack the ability to reproduce them. Then you can make the AdMech arc here.
Or maybe you don't have to. Unless the chapter is hell-bent on replicating these jetbikes then maybe they're just chapter relics in a sense and highly praised. Would give those who are permitted to ride them a lot of respect within the chapter. Plus, even without the STC, I'm fairly certain it wouldn't be impossible to imagine Tech-Marines being able to repair most damages. Maybe this is where the AdMech comes in, when a bike is so badly damaged it is sent to this friendly forge world where they are able to repair it.
The chaos gods abandoned Horus most likely because they saw the can of whoop ass coming their way and wanted out of the way so as not to get fucked up!