Only partly true. The idea of insurance is to pool SIMILAR risks. It isn't to have low-risk people subsidize high-risk people.
As examples, fire insurance premiums vary with the riskiness of house construction, the nearness to a fire department/hydrant, etc.; auto insurance premiums vary with age and previous accident experience (mine went up when I had an accident); life insurance premiums go up if you already have a chronic disease at the time you apply (I got hit with that when I bought insurance for my first wife, after she was diagnosed with kidney problems).
It's entirely reasonable for insurance companies to demand higher premiums for people who belong in higher-risk pools. The idea that they may be denied insurance completely is simply a red herring. They may find the higher premiums unaffordable, but that's not the same as being told they're absolutely uninsurable.
"It isn't to have low-risk people subsidize high-risk people. "
That is EXACTLY what it's about. That's how premiums can be affordable. And quite often low-risk people end up with some illness which costs huge amount of $ to treat.
"They may find the higher premiums unaffordable, but that's not the same as being told they're absolutely uninsurable."
Yes, it is the same thing.