Only some of the STN domestics are away based, mostly GLA and EDI, but by no means all. In traffic terms STN is still bigger than LPL, GLA, NCL and SEN, so it's not insignificant as it may seem.
EZY, a company that's success is founded upon its dominance in some the largest, slot constrained airports in Europe, would preserve SEN above all else because of it's small size and convenience, I think not.
Frankly, I dont think EZY ever expected FR being able to get into SEN. I wouldn't be surprised actually if EZY didn't renew a contract with SEN (in 2023?) when the current expires. On the subject of LTN, which is what we ought to be talking about here, wouldn't have thought any uplift in LGW would cause too much damage. More likely aircraft would be sourced by other less profitable routes across the network.