It is speculation at this point what really changed Farage's mind, but this article in The Guardian states that:
The Brexit party leader claimed he had changed his mind about fielding candidates in 317 seats held by the Tories after Johnson released a video pledging to take Britain out of the EU by 2020 and to pursue a Canada-style trade deal.
The abrupt nature of Farage’s reversal prompted claims from Labour, the Liberal Democrats and the Scottish National party that he and the prime minister had struck a secret pact in favour of a hard Brexit, which both sides denied. Farage said he had been offered a peerage by the Tories as recently as last Friday but claimed he had turned it down.
At the rally in the County Durham seat, which voted 70% to leave the UK, Farage said he had concluded that if the Brexit party stood a candidate in every seat, it could split the vote and usher in dozens of Liberal Democrat MPs, in turn creating the circumstances for a second referendum.
Farage said he had been reassured after Johnson said he would not accept an extension of the transition period for Britain leaving the EU beyond the end of 2020.
Also of note, in that article pollsters are skeptical that Frage's new plan will actually work exactly as he says it would, because he is still splitting the vote in the seats where Tories need to make inroads.
But regarding "withdraw" candidates, it does look like at least some of those actually existed, and some are pissed off with Farage now:
Brexit Party MEP Alexandra Phillips, who was due to stand as a candidate in the Tory seat of Southampton Itchen before Mr Farage’s announcement, has declared she will “not vote at all” at the election. [She tweeted:]
I will be one of millions of people who will not vote at all in the General Election. That breaks my heart. I have voted in every election since I was 18 and been involved in politics for over a decade. And I have been disenfranchised by my own party.
Robert Wheal, who had been due to fight in the Arundel and South Downs constituency, said Mr Farage was “finished as a politician” following the climb-down.
He tweeted: “All that Farage has exposed is his duplicity to so many supporters who had put their faith in him.”
While two tweets are far from the extra 300+ candidates you ask about, it does look like at least some of them were not entirely bogus.
Actually, the Guardian now has a subsequent article, mentioning more details of the former candidates, and naming at least three more:
Darren Selkus, who was the candidate for Epping Forest, said Farage had “betrayed my incredible volunteers and thousands of constituents who will have no one to vote for” by pulling out of all 317 Conservative-held seats.
In a statement on his local party website, Selkus said that as soon as Farage made the announcement at a rally on Monday in Hartlepool, he and other ex-candidates were immediately locked out of their Brexit party emails and supporter databases. [...]
Julian Malins, a barrister who was due to stand in the Tory-held seat of Salisbury, tweeted: “I thought I had enlisted in Caesar’s army but it turned out to be the Grand Old Duke of York’s.” [...]
Claire Mowbray, who was to have taken on Theresa May in Maidenhead, tweeted: “I can’t tell you how disappointed I am.” She added: “I will be closing this Twitter account.”
But insofar I couldn't find out how many more candidates there were in total.