And for some people, the math would probably work out ok. (This tweet's longish).
Take an imaginary 48-year-old couple with 2 kids, who makes 450% of the poverty line. They would have gotten a premium tax credit of $1,200 a month last year, or $14,400 annually. Assume that goes into an FSA.
The maximum annual out-of-pocket spending for a family on an ACA plan is around $21,000. In an absolute worst case scenario, where two people got very sick, you'd be looking at $7,000 out of pocket, plus whatever you're paying on the bronze premium.
More likely, they're not paying much out of pocket at all beyond their premium...