Farage has always not been great at General Elections. This 500+ standing over some ultimatum will alienate some of his voters from his party. There were already signs of disagreement within his own party when he initially said the deal was shit. His own party members snapped back. The only ones who will be on board with this move would be the politically unsavvy who say things like "country loyalty over party loyalty" without looking at the numbers. This seems like an attempt to get him back into the news after disappearing post Euro elections.I think Farage and TBP spent their political capital on the MEP elections. Everyone I know who voted them in the euro election is planning Tory this time because that is how Brexit is going to get done.
The problem TBP has is they peaked then disappeared. They appeared to a fanfare with a whirlwind of media coverage and people flocking to them. They ate the UKIP vote and some of the Tory/Labour vote. Then tried to keep the momentum going in the by-election, lost, and the promptly disappeared (possibly media strategy to ignore them or them failing to live up to the promise of their initial fanfare). Farage has done a poor job keeping them in the national consciousness and that will hurt them.
The fact is his party is a single issue party, as have all the parties he has led. A very important issue, but single issue nonetheless. As important as Brexit is, people also have other concerns that need to be dealt with, be it the NHS, Crime, Housing and so on. Every time UKIP (and i would not be surprised if Brexit Party is the same) is pressed on these issues, their answers are never highlighted or pushed because all they seem to promote is their single issue. They will never be a major party in the UK outside of Brexit and will never have a real presence in the UK parliament.