US health insurance

I am facing a potential cancer diagnosis. My partner and I are not legally married at this point. I am a SAHM and not currently insured. We have always paid out of pocket for my medical expenses. If I receive an official cancer diagnosis, our plan was to elope and I would be added to my partner’s health insurance plan. He has BCBS, I think it’s called FEP (he is a federal employee).

Am I correct in understanding they would cover any treatment? I don’t think they can refuse to cover “pre-existing conditions” anymore.

Any thoughts? Insight?

Edit: We don’t want to elope unless we have to. We would prefer to wait and have a real wedding. Yes, I realize we could still do that later down the road, but it isn’t what we want.