I reckon the truth is somewhere in the middle.
The AFL clubs can't force their players to certain clubs, but they certainly can coerce to a degree by way of friendly conversations with SANFL clubs about who'd they'd like to see there and why (whether a SANFL club does as the AFL club would hope is another matter).
Not many SANFL clubs want to get either of the AFL clubs offside, most find it more beneficial to have good working relationships with them rather than having a situation where they are working against one another.
Just my own thoughts, might be way off the mark.