TB you’ve been at this a long time so I know you know this, but sadly most parents don’t.
The main reason coaches, especially at the larger clubs with many teams per age group, play state cup is to use it as a recruiting tool. So bringing ecnl players is part of the marketing package. Win, claim the title, and bring more kids - naive ones who think winning state cup is special, when there is national cup at the age group and ecnl/DA teams that don’t participate - into the club
There were other years I experienced and heard from friends that their coaches not only lamented national cup but strategically lost to exit early and move on to the next year.
I would guess in the Slammers case, that they brought 4-5 in to replace those who will be cut anyway. Some coaches will tell kids, you can come to state/national cup, but you won’t play.
Slammers, and other large clubs likely could use some other ecnl Players in National Cup, except their season is going again then.
In the end, as much as parents don’t want to realize, club soccer is business, clubs will take anyone’s money, and for the most part these cups are worthless.
But they sell a ton of sweatshirts, parking spaces, and they enjoy their take from the hotel fees.