It has to do with the encysted stage of the parasite. The copper only kills the parasites in their free-swimming stage, when they burst out of their cysts in search of a fish host. Once they find a fish, they burrow in and are protected by the fish itself to feed and mature.
After this, the mature parasite drops off of the fish and settles into the substrate where they form their protective cyst-cocoon. That’s where they reproduce, and the cycle begins again. The longest known strain’s lifecycle was 72 days... [emoji15]
But, most complete their entire life cycle within 30 days hence the recommendation. If you wait 14 days the parasites on the fish have a chance to mature and fall off the fish, so if you transfer them away without lowering the copper you are leaving the newly formed cysts behind.
If not, you have to wait until the new cysts that were formed burst, and all of the free swimmers destroyed/lifecycle broken so that they don’t just become reinfected after you lower the copper. Technically speaking, even 30 days is a bit short of the possible 72 day lifecycle (why it’s recommended to leave a display fallow for 76 days) but it’s hard on the fish to be in copper much longer than that, so 30 days is a low- risk, happy medium.
In all honestly, it’s much better to use the transfer method. Then you can be sure the fish are fully eradicated and it’s much better for the health of your fish because of the shortened copper exposure.