I'm guessing he put the board in and then started the oven which took a while to preheat. Industrial reflow heats up the board gradually based on the solder pas

I'm guessing he put the board in and then started the oven which took a while to preheat. Industrial reflow heats up the board gradually based on the solder paste manufacturer's suggested heat profile or experimentation/trial and error (e.g. when dealing with small pitch BGAs). 250C for even a few minutes would do quite some damage even to the fire retardant laminate and smell horrible.