A wet Midsummer can matter to an autumn forest only if its water remains in the ground through the hotter weeks that follow. The records support that mechanism in broad outline, not the calendar rule.
A myrstack can be a useful sign of the forest structure around it, but it is not evidence that fruit bodies will appear nearby. The old rule notices a real association and then asks it to carry too much.
A folk rule with a long history, a plausible-sounding mechanism, and thin evidence. Worth understanding properly rather than dismissing or believing.
Four to nine days after a good thunderstorm. Nearly every forager has heard it. Here is what could actually be behind it, and what the lightning data lets you check.