I'm not sure where you have heard that chatter as London is not included in the India 5th freedoms so unlikely to allow Scoot to fly SIN-India-STN. Current 5th freedom bilaterals specifically state London and the US is not included.
Obviously nothing stopping Scoot starting SIN-STN as a stand alone route, but SQ have also mentioned they don't want to dilute their London yields.
It's worth pointing out that the SQ51/52 serves 3 different purposes, SIN-IAH-SIN, SIN-MAN-SIN and MAN-IAH-MAN so should stand up on those 3.
The new Scoot routes are primarily aimed at India-Europe and Singapore India, so, won't be competing directly with the SQ51/52 in the traditional sense.