No it isn’t. That date comes from an FOI request document which includes a line saying the earliest possible date would be 4 April, however no actual date has been confirmed.Is Superloop route SL12 (Gants Hill to Rainham) due to start tomorrow? I've seen from a few online sources that Saturday 4th April is the start date but nothing from TfL....
The current time floating around at the moment from more reliable sources is September. Bearing in mind the new batch of electric buses due for the route have not been built/delivered yet, and standing space still needs to be sorted at both ends.