Travel health insurance claim denied. What's going on?

Last November I went on a trip to Mexico and became sick after a few days. The day before returning home I arranged for a doctor to come see me, who came with an ambulance. She only spoke intermediate English and there was miscommunication. I explained my symptoms. They took my vitals and my blood pressure was very elevated. She recommended I go with them to the hospital so I did. No further assessment was done there I was only given some medication and released a few hours later with a slip which stated hypertensive crisis- *which was a clear error because my blood pressure readings- which are stated on the slip -are well below that level* It did not mention my other illness. Because I had a cardiac exclusion on my insurance because of outstanding tests I was concerned that would be an issue. But after months of waiting I received an email stating that my claim was being denied because I had withheld information.

I do not remember details of the questionnaire when applying but I recall it walks you through things and you respond accordingly. They list several things that I supposedly withheld, but most of them I do not recall being asked about. I did overlook to include part of a mental health diagnosis. They included a history of substance abuse, which was no longer relevant (they even mention the alcohol abuse is "in remission") the opioid use is ancient history. Cannabis dependency was partly true . Other things mentioned like sinus pressure and deviated septum and obesity make me question where I would have given stated those.

It incorrectly states that I had prostate surgery. It mentions my bladder cancer, but I would have mentioned it or the bladder surgery if they had fallen within certain time frames or been asked about, but I just don't remember. There are a few others as well. Since it has been so long I can't swear that none of these had a place in the questionnaire where they should have been mentioned outside of where one responds yes or no and then adds information where a yes is given, but I am certain that would be minimal!

The diagnoses it states that I included are primarily ones that I take medication for and probably came up in that context.

Why are they including diagnoses that are no longer relevant and or would not have come up in the questionnaire? Were they just fishing for a reason to deny my claim? I'm hoping I can fight this. What should my next steps be?

I know I can get a letter from my doctors stating that some diagnoses are no longer relevant or misstated.

Also, I never paid out of pocket and don't even know what the dollar amount is and that subject has never been brought up by them. What is likely going on with that?