Double digit premiums sound ridiculous, even for a male in their 20s. As a data point, I'm a <30 male, never been to a doctor with no health issues that I can tell, and my HDHP premium is $310ish, last year it was $275ish, but that's using healthcare.gov so completely unsubsidized, and I have a deductible of $1,500/$2,500 and out of pocket max of $3,600/$6,550 for their tier 1/tier 2 providers, respectively (whatever that means).
Since it was grandfathered in it doesn't have to offer all the benefits of the ACA.
I'm not sure if a grandfathered plan can have a maximum payout, but if it does that would explain the low cost.
Years ago I had a plan like this, it was cheap because they'd only pay a maximum 100k per year.