Problem is, the Astartes creator specifically calls out Warhammer as being the inspiration behind his shorts, both in the videos themselves and on his YouTube and Patron pages. The fact that he would then turn around without Games Workshop's permission and let someone profit from their IP is bound to be problematic. We're way outside of the usual grey area Chapterhouse was in where they could claim it wasn't GW's universe, just one with similar visuals...
Except as long as it is all produced in a legal fashion, it doesn't matter in the slightest what the "inspiration" for something is - yes, seriously. As long as they don't use trademarked (or other IP covered) material for the sales (and the court in the U.S. has already stated that they can't claim basic shapes such as the shoulder pauldrons as trademarkable), the legal ground is shaky - a case must be made and agreed to by a judge to hear arguments.
It's not any different from an aftermarket light kit being sold that transforms an automobile light from a standard to LED set up, even with the official kit being sold by the manufacturer.
The Astartes creator already covered himself by doing the whole "fan-made" thing and specifying he intends no usurpation of claim by using trademarked items, etc. Kromlech is covered by not using any trademarked terms or material. There's very likely no "joint" infringement - you typically have to make cases against individual material infringers directly, or you have to be able to show some kind of conspiracy to mislead the consumer that something is official, etc. If the material being sold is not trademark covered, then there isn't a way to do that, because there's nothing saying "this is official product based on X trademarked item". Also, you typically have to bring the case in the jurisdiction that the infringement occurred in originally, not distribution points/destinations, so depending on how the IP laws work where it happened, you have to satisfy the elements of that jurisdiction for a hearing.
Seriously, if you guys feel so strongly that this is an issue, go report them to GW
's attention via any means you might have - GW
is the only entity that can determine if they want to pursue a case and would have legal standing in the situation - about the only thing that might be an issue is the specific use of the term Astartes (not sure if that specific term was called out as unenforceable - Astarte is a historical name, but not in conjunction with space-borne infantry, or if it is able to be enforced due to use to clearly refer to a specific type of "space Marine" - specific use/reference is often a key factor). All of us on the outside don't mean squat.
You are allowed to be inspired and create something derived from someone else's IP - and if things aren't trademarked or can't be trademarked, that can mean being able to sell parts that are identical to a manufacturer's parts.