Altough oden strength can survive boiled in oil..
if those unconcious yonkou can survived drowning in magma, it is too OP though..supposedly it can cause death..
Not a fan of suddenly there's a magma below though..but i think as the plot story arc is moving from old yonkou era, maybe oda purppsedly put the magma as a story shortcut to end them and can move forward to other villain..
bcoz if the 2 old yonkous survived, they might be still to strong, and can impact the upcoming villain : gorosei, blackbeard, shanksVSdragon? plot story arc..