there is nothing to stop the formation of the event horizon. Time dilation is not a local phenomena, so locally a particle falling toward the event horizon just passes through it without even noticing.
This is not a case of assuming the conclusion, it is a rebuttal of a specific argument. The argument is that there would be infinite time dilation near a horizon. The rebuttal that time dilation is not a local phenomenon is valid.
The argument assumes the horizon and tries to show proof by argumentum ad absurdum that the horizon does not exist. The rebuttal is perfectly entitled to assume the horizon (required in fact) in demonstrating the non-absurdity of the result.