It does indeed seem a strange strategy. Having a foothold in London is obviously key to the airline going forward so why leave LGW & to a lesser extent LTN with nothing lined up to replace it immediately?
A LCY operation, if true, represents a significant shift towards business travellers & presumably higher fares that often accompany LCY flights in comparison to other London airports.
Also, using LCY & SEN seems a slightly strange combination. Will a combination of both offer passengers something akin to what they were used to at LGW?
As EI-BUD rightly points out, by the time these new flights begin, regular passengers will have found alternatives, be that BA, easyJet etc so why go back to FlyBe? All in all, FlyBe's London strategy does on the face of it seem to be quite muddled.