Realistically speaking, since this is in fast charging only, and most people don't let their phone battery drain to 0, I think it's unlikely that 3rd degree burns will be an issue within reason. The iPhone 15's reported to take about 30 minutes to charge from 0 to 50%, so if I assume the other 50% also takes 30 minutes, then really, someone won't be hitting 45 minutes unless they're charging from below 25% to full.^[I know charging isn't actually linear progression, but I don't really wanna do the math.]

That said, 3rd degree burns may be an issue once you slip a case on it that insulates the phone, making it yet hotter, and 2nd degree burns will be more of an issue too. I'd be interested in seeing what the peak temp is during fast charging with a case on, especially one of the thicker cases. If it internally gets to more than 140 degrees Fahrenheit, there's absolutely risk that it could explode, and even if it's not doing it externally, internally it may be nearing those temps.