If the guy is blown the luff of the sail will go forward and it is easier to retrieve it with sheet because the pole isn't in the way. Usually the problem arises when the boat is sailing high on the wind and the spinnaker is strapped to the forestay. With the guy blown, the spinnaker streams off to leeward. This is how spinnakers are often doused on big boats, the foredeck releases the guy from the spinnaker while everyone tries to get it down behind the mainsail.I always thought the best and fastest way to de-power a spinnaker is to completely release the guy. Then the spin would fly like a flag, the boat would stand up and you can recover. Some, maybe on this forum, have said this is a dangerous practice. I would like to hear why? (I'm not referencing the video)
When the sheet is released, the streams off to leeward but does so from the pole and not from the masthead. This makes it more difficult to retrieve the spinnaker because it is forward and the pole is in the way. The guy always has more tension on it, so it releases faster.
With this said, when the hits the fan in a broach, depowering any way possible is the goal. Recovery is easier and faster when the guy is blown.