KtP recommends that primarily for the benefit of off-site guests. It means that a RD at a park with EMH for an off-site guest is behind the on-site crowd. Furthermore, the on-site guests tend to pick that park as their park of choice in order to use EMH, which means MORE on-site guests in that park than other parks.
So, the idea is that if you avoid the park with EMH, you aren’t fighting against a higher level of on-site guests.
If you ARE an on-site guest, then going for RD at the EMH park gives you an advantage over off-site guests, as well as on-site guests who aren’t there for RD. But, once EMH are over for that day, you might want to hop to another park to avoid the now elevated crowds.
I’m not sure if there is necessarily a good way to determine if this is actually true in practice, or just anecdotally, though. Because off site guests would be more inclined to AVOID the parks with EMH, which distributes the crowds to the other parks. It might all just balance out in the end, depending on the ratio of on-site to off-site guests.
There are other factors to consider as well, such as days when parks have parties. For example, when MK has MVMCP or MNSSHP, hours are significantly shortened for the day, which makes people less inclined to choose that park to go to UNLESS they are going to the party. This means the crowds could be lower, making lines shorter. So, while you have shorter hours, you also have shorter lines. Which means, once again, it kind of all balances out.