I’m not an expert in fire safety. I don’t have an opinion on how safe it is to ignore the flame retardant layer, but your argument is at least sane enough to pass the smell test. If the author had said that, I’d have been satisfied.

They didn’t even do that, though. They said “I don’t like my options for fire retardant, so here’s how I built a mattress without it”, with zero justification for why they thought it was acceptable, zero assurances that they put any thought into it.