Which bit are you having trouble making sense of?
B787 massively delayed so Boeing might want to make up for it by offering a favourable deal on a new B737 model (plus a coup to switch an EU carrier to Boeing), earlier variants (-200 and -300) which Monarch used to fly anyway in the 80s/90s (and the B757 for over three decades) so it and Monarch Group's engineering co (MAEL) are no strangers to the type and operating Boeings (MAEL is anyway a B787 GoldCare MRO).
Shortly after, ABs with winglets became available when leases were up on older aircraft so why would the airline not take advantage of the fuel saving on offer for its AB fleet in the near term, before the B737s arrive in 2018?