I heard that same thing years ago. I've never had wpm (Knox on wood) so I've never tried it. Have you tried it personally?
Yes, I have had PM three different times, one at each of my spots. All years apart. The first time it got me was about a week before harvest, during my second ever grow. I didn't know what it was and thought my entire crop was ruined! 4 lights, 6 pounds and all covered in PM. I was distraught. Then I remembered.......THE INTERNET!! I cut down at the end of the run and cut all the buds from the stems. I then took all the bud home and filled my bathtub with water. Poured in some H2O2 (can't remember the ratio, but I want to say close to a gallon for about 3/4 full standard bathtub) and dumped in all my bud.
Now, this may sound crazy or counter-intuitive, BUT what did I have to lose?? Immediately the water started bubbling. The H2O2 was eating the PM!!!!! I left the bud in the solution for about 5 minutes, mixing constantly. I then drained the tub and rinsed all the bud several times. Not knowing what to expect. It looked great!!!!
I then threw it all on drying racks. It took about a day longer than usual to dry. Once it was dry, I trimmed it up. Looked fantastic, smelled amazing, still had all (or most of all) of it's trichomes and tasted/smoked great!!!
I ended up spraying those rooms down with bleach and then ran sulfur after the bleach had dried for a couple days. Never had PM in that spot again.
The next time I had PM, I knew what I was dealing with. I immediately ran a burn for four hours during lights off time for three days. PM was gone, never came back.
The last and most recent time, one of my mother/veg rooms got it. Ran a sulfur burn for four hours/one week (much larger area, about 1500 sq ft) the PM was gone......except for some of the babies I had just moved to a flower room. I saw signs of PM in that room about 2 days after filling it. They hadn't gone into flower yet, so I pulled all leaves showing any damage and did a sulfur burn for 3 days. Problem solved. I did that run and 2 subsequent runs in that room and not a spec of PM.