I don't know, she didn't explain but it was an immediate response when the subject of Boris Island came up. Given the timescales involved, maybe she's referring to the TB no longer being up to the job if an airport was built in the estuary.
Update:
Just reading up a little I found this which mentions the project and the inclusion of a new flood barrier.
https://en.wikipedia.org/wiki/Thames_Estuary_Airport
My preference would be for a whole new airport but it's not going to happen.