I remember a discussion about this in uk.railway many years ago, it was basically a future proofed layout that stopped dead under the higher level approach to Spitalfields coal depot. They aligned it so that with a suitable tunnel dug under the GE Liverpool St lines it could then head off in the right general direction, but then IIRC (from distant memory) it couldn’t be built north of the GE, because by then housing had covered any practical route.
I suspect it was all about possible freight services rather than passengers though.
NLS side by side 25” map shows the stub end:
It seems to me the alignment shown in the Atlas is a bit more optimistic than in the old OS map, ie it’s as though it’s been drawn aiming directly for Cambridge Heath, a bit further round than the angle on the map?