The ability for Banshee to negotiate the X/Y split is an assumption. No one has so far stated that Canonical was willing to negotiate on the terms of the split or that Banshee developers failed to inquire if Canonical was interested in negotiating. The only statement on record that we have is from the Banshee camp that Canonical presented them with 2 options and they chose the option they were most comfortable with.
The statement's so far on record are not written in such a way as to give insight as to whether Canonical was receptive to any efforts to negotiate something other than those 2 options or if any such efforts from Banshee devs were floated. If you can point me to something written from either a Banshee dev or from a Canonical exec that suggests otherwise...please providing me with a reference for me to read over.