No tsunamis in Wivvy please chap, that's one of my fishing spots and we've had more than enough rain locally already!
I think the old adage dates from skep beekeeping, which actively used collected swarms as part of the system. Whereas early swarms could being in a useful crop in the same season, a July hived swarm was unlikely to bring in honey that year and would need additional feeding to successfully overwinter. A lot of cottage skeppists could not afford sugar to feed bees, so a late collected swarm was considered a financial liability rather than asset and not worth collecting.
These days the economics are a bit different so whilst losing a swarm in July is a pain in the butt, collecting someone else's can be a late bonus