What sort of confidence does this warrior have in the first place, to be able to pull off such a statement?
After listening to Joshua's plan, Brandon was stunned by his ambitious action. A Gold-tier warrior against a Chaos Corrupted Black Dragon? That sounds more like a suicide mission. Ordinary Gold-tier teams would not take up that task. Joshua is just a single warrior of his own, where does that confidence come from?
The surprising part comes later as when Brandon thought about it, Joshua's plan was not wrong in the first place.
No matter how they plan, or how many plots were hidden behind the agenda, to stop this beast tide, they had to kill this black dragon. Screw the dragon! Infected or not, they had to confront each other at the end of the day.
If that's the whole case, was there even any necessity to further think about what plots were behind the scene in the first place? Grab your sword and slay the beast!