Just an idea, maybe they are trying to get that route of energy recovery banned?
Pulsing the the MGU-H at 20-40Hz will create vibrations. Maybe that has something to do with all these MGU-H failures they had. Developement might be expensive and time intensive.
If they publish this information they are probably sure the other manufacturers know about it and also do it.
Let's just assume the others are better at it right now. Although storing the energy from the MGU-K in that way is not against the regulations, it's probably against the intention that is behind the regulations.
If Honda can get the FIA to ban this practice they just got a step closer to the other manufacturers in terms of power and reliability. And it would cost them nothing, compared to expensive developement. And if the others really also do it Honda can't loose anything by publishing that information.
So it might just be a clever and inexpensive try at closing the gap to the other manufacturers.