"They" mean YOU. DirecTV won't be doing this for you.
so they would have 2 DECA modules than one for the HR21 and the Slingbox
One DECA powered/connected to the HR21, & another coax feed [splitter] to another DECA that is powered from a PI, and going to the Slingbox.
"Plan B" would be to use a switch from the DECA and then feed both the HR21 & Sling, but this isn't what DirecTV will do or wants.