Instead of contesting 600-odd seats, Nigel has decided to refrain from contesting Tory seats. This relegates the Brexit party to the role of spoiler for Labour; all they will do, is divide the Labour vote, increasing the chance that the Libdems (or the Tories) will win those seats.
This is a win-win-win.
First, it's an admission by the Brexiters that their ideal of a crash-out chaotic Brexit, isn't as attractive as a Brexit with an ongoing arrangement with the EU.
Secondly, it increases the chances of the Libdems, by dividing the Labour vote, and giving clarity to people who like the promise of the Libdems to revoke article 50.
And thirdly, and most excellently, it will add to the punishing blows that the racist Labour party will get at the polls. After a sound defeat, the Labour party will throw out the weak-as-water Jeremy Corbin, and promote someone else, who will (hopefully) take the very simple steps that will eradicate racism in the party.
The most racist party in the country is the BNP. They had a demonstration recently - twelve people turned up. The party is dead. The second most racist party is UKIP; their adoption of "Tommy Robinson" and departure of Nigel sealed their fate. In the next election, they'll poll worse than Lord Buckethead.
But at least they know they're racist. The Labour party is still in denial, despite it having been proved to them again and again.
So - with this move, Boris should do rather well. Which isn't ideal, but better than most of the alternatives. I'm more optimistic about the future of Britain than I was yesterday.