I understand. The other side of that coin though I guess is that knowing this might give some more confidence in using diesel. ActiveRecord was the first ORM I used, and knowing now that diesel is "from the same stock" (for want of a better phrase) gives some extra confidence in what it can do / where it is going.
Not that I was not confident in it before...
I don't see this particular case of someone who spent a significant amount of time coding core Rails OSS library being seen as a negative though, quite the opposite. If anything it would be an excellent measure of the language for that particular use case assuming any competent developer would seek to operate in the constraints of the language rather than shoehorning an existing implementation into something that doesn't fit.
The bigger question is whether the ORM label itself is limiting or can be understood to fit into a broader scope of implementations.
I agree that the "I make Rails" thing is a double edged sword. I would hope people would see it as an indicator that I have a lot of experience and lessons to learn from, but I think many people see the opposite.
[0] https://github.com/tildeio/helix - When Helix can handle that complexity (if it can't already)