I'm guessing that the reasoning here is that UDFA signings will essentially mostly occur when the signee is seen as a replacement for an inferior player already in the club's system. This approach would be consistent with the new, arbitrary limit on the number of players an organization may have under contract (180, in season; 190, off season), and the halving of the size of the Rule 4 draft. The intent is to reduce the number of people signed, to reduce the number of people under contract. So the Phils cannot sign fifteen UDFAs, unless they trim their "Domestic Reserve List," by releasing other players, to make room. They can defer making room for the draftees - bring them to the Complex after August 1 and further evaluate them until after the World Seris - but they cannot arbitrarily expand the number of players they bring to the complex, and evaluate for a couple of months before deciding who to keep, who to cut.
I'm sure this isn't MLB's intent, but this may actually be a bit beneficial to independent teams and leagues - assuming there's an economic model that works for such an enterprise, preventing MLB-affiliated organizations from tying up essentially unlimited numbers of players may lead to independent teams/leagues actually having some viability. One can hope.