That doesn't mean the report isn't legitimate (being cluster-admin with no authentication is generally considered not to be a good security idea, in fact it's about as bad as it could get without putting the insecure port on the Internet), but that bug bounties aren't architected to accept that kind of issue. The challenge with this is it means that bug bounty researchers won't look for that kind of (legitimate) security issue as they get to know the programs often won't pay out.
Personally, I don't ever report bug bounties for the money reward, but so I don't get shouted at by companies when I write a blog or do a talk that covers the issues :) In this case I was a bit annoyed that they combined telling me it wasn't a bug, with asking me not to mention it publicly for 6+ months (IIRC they credited me like years after the fact).
You would prefer this company to have made a different strategic choice about what to spend engineering time on, and that's fine. But engineering cycles are finite, so whatever time they'd spend configuring K8s differently is time they wouldn't be spending on some other security goal, which, for all we know, was more important. Software is fathomlessly awful, after all.