Will reserve further comment until the plastic inserts function is determined, tho my guess is that its design is integral to the lugs performance/function.
With regard to that, the only reason I can see for the plastic part is to keep the metal rod insert in place. Traditional lug housings are bigger, and their metal insert can be put in place from underneath - the inside - because there's enough space to manipulate it. As a much smaller housing, the Signet lug doesn't have the same access from inside. The lug is sort of 'open', the metal insert is put into place, and the plastic insert snaps into place to hold it, and round out the shape of the lug. Oh, they probably could have made the 'plug' out of metal, but there's more cost involved for material and plating. Even a dime or quarter more apiece adds up fast, and cost-savings is an important selling point for this line.
I know it seems like I'm overly defending different aspects of these drums, but it's only because I'm apparently the only person on this forum who has first-hand knowledge and experience with them. And until someone else has a kit in their hands, I believe I'm uniquely qualified to talk about their sound, construction, or inspect a lug close-up if necessary (other than not being able to take it apart!) I've been very upfront about the pros and (perceived) cons of these drums, and the plastic piece in the lug is not going to be an issue.
I can find only one possible objection to these drums, and that's the kick spur length not adjusting, and the drum angled too high for some. That is, if there's one thing I would suggest they change, it would be that. But I'm happily playing my Signet kick angled as-is, I have no desire to modify or change out the spurs.
Bermuda