I did not know the plan was to cut across on SR 536. Disney Springs is indeed close to the I-4/SR 536 junction. There's an extended exit ramp from that interchange directly into DS, which they could easily follow to a station on the West Side of DS.They planned to take SR 417 to 536 to I4 which already gets them to the vicinity of Disney Springs.
ETA: This map from the the Orlando Sentinel in Jan. 2019 does not show using 536:
Source: Brightline to start building rail from South Florida to Orlando in March while it rebrands as Virgin Trains
Also in June 2019: Virgin Trains officials to break ground on high speed rail system in Orlando