Why a secret, it's a pretty obvious way to increase (air) capacity at
much lower low cost/impact than a third runway at Heathrow.

It's the increased airline capacity bit they're trying their hardest
to be dishonest about.

Scrapping a third Heathrow runway and building HS2 instead was
supposed to be a way of reducing internal flights.

In fact, HS2 will actually allow for the additional airline capacity
the third Heathrow runway would have provided, it's just that the
third runway will now be in Birmingham.