It is marketing spiel. Watts are watts, and 'cab watts' mean pretty much nothing. A 400w valve head isn't going to be more likely to bust stuff up than a 400w SS head, maybe less due to the compression. That said, they can totally still blow a cab rated at more than 400w, because that rating isn't the part that breaks.
You just need to match the impedance, and pick a cab you like the sound of.