We don’t much care about public oaths these days. But in the era of illiteracy, a man was literally only as good as his word. A public oath was admissible in court (why they were talking so loud), and it was legally binding.
That he was pledging public loyalty to England, while simultaneously helping the insurrectionists, might be lost on younger viewers. There were no situational ethics then, just…ethics. And because the Patriots were warned, they were able to set up this ambush.
From a military perspective, you can see just how they were able to set their lines along the most obvious avenue of approach. I refer to it as a ‘reverse-ambush’ not because the wrong team got ambushed, but because the group that was supposed to be ambushed (the blues) were waiting for the ones who thought they were going to be the ambushers (the reds). The key is that the Patriots weren’t in a defensive position (dug in to defend, or in this case, barricaded). They were set up to attack.
You can already see on the battlefield the colonials moving further toward guerrilla-style tactics. Poorly-trained militia wouldn’t really know that much about columns and lines, but would know how to hit an animal racing at 20 mph the other way through trees and scrub. It‘s also obvious the redcoats are having to adapt, as well. Well-formed lines don’t work well against snipers in trees.