Yeah, I'm honestly not sure what the other candidates are supposed to do.
The other candidates are running for President because they have specific political goals that they want to sell to the public and achieve. Also, they care if they win.
To the extent that Trump has actual policies he wants to accomplish, he apparently either figures he doesn't need any allies to achieve them, or doesn't understand that it doesn't work like that in politics. It's debatable whether he even has any such causes or issues that he honestly cares about. If so, not many.
Trump wants to be President not to lead the government to do certain things, but because it's the "top job." He doesn't care if he wins, really. If he loses, he's still rich and famous (far more famous than if he hadn't run, actually.)
Because they have such different motivations, Trump can say whatever he wants, but other candidates can't come back at him in the same way. I don't know how that dynamic changes.