Let me put this differently. At the time Roberts signed for Souths were approximately 157 days remaining in the 2018-2019 contractual year. This represents about 43% of a whole year. So if Roberts is paid 200k for that period, it is the equivalent of $465k annually. If he gets paid 250k by Souths, that is the equivalent of 580k annually.
Now if GI was on $1m, and he retired on 15 April, about 45% of the contractual year had elapsed, meaning $450k would be charged against the cap. This leaves $550k available. So even if Souths paid GI $300k for gardening services, there is still $250k left over to cover a salary of 250k for Roberts (for 157 days work).
I don't get what your problem with the arrangement is.