But that is a bit of a joke. Because - as he surely knew - the only credible challenger candidate will be the LibDem candidate, and LibDem policy is to oppose all airport expansion in the South East.
So the by-election will much more likely be a mini referendum on whether the country was right to choose Brexit, because Goldsmith was and is a Brexiteer and the LibDems are in deep mourning for our lost EU membership.
And with 'Richmonders' overwhelmingly pro-EU, Goldsmith's grip on this seat is by no means super-glued.
How has it come to this?
Well it is because May lacked the bottle to put up a Tory candidate to challenge the now "independent" Goldsmith, even though Goldsmith was savage in his criticism of her decision to sanction Heathrow - because she was advised that splitting the right-wing vote would let in the LibDems.
So what the Goldsmith resignation and Heathrow shows is how perilously weak the government led by Theresa May is, with its tiny majority of 10 (post the Goldsmith defection) and Tory MPs taking lumps out of each other on Heathrow, grammar schools and - above all - the true meaning of Brexit.
The big fact about the May administration is that she is a strong character in charge of structurally feeble government - a ruling party riven by personal and ideological divisions, almost all of the animosities stemming from Tory MPs' competing religious beliefs about what our future relationship with the EU should be.
That extraordinary weakness is visible in May granting a licence to Boris Johnson to thumb his nose at the boldest decision she has taken so far, the runway construction (and the identical licence granted to the education secretary Justine Greening, who is so far not using it as conspicuously).
None of this feels sustainable - given that May faces challenges of a complexity and magnitude no British government has faced since 1945, namely how to reconcile control of immigration with an economic and trade policy that doesn't impoverish us, and how simultaneously to keep the United Kingdom intact.
So in spite of her protestations that she does not want an early general election, it is increasingly difficult to see how she can negotiate Brexit and the maintenance of Scotland within the union unless she wins a personal mandate by going to the country - and probably as soon as next year.
The alternative will be chronic instability, endemic crisis, which she won't like and the country will rue.