I don’t think there’s a single right answer. If you don’t get any reply to your PR, I think it’s reasonable to try GitHub username-pinging the user that was most recently active on the repo.
This can sometimes be a bit misleading. GitHub sets your organization membership to “private” by default. You can make your membership public, but you need to do it manually. So it’s possible that the users you’re looking at are actually members of the JuliaMath org, but that their memberships haven’t been set to public.
But also, it is possible for someone to be a maintainer of a JuliaMath package without being a member of the JuliaMath org. Those people are listed as “outside collaborators” in the repo settings, but I think that’s only visible to org owners and repo admins, so members of the public likely can’t view that status.