It's a bit more complicated in Colorado (and many other Western states). His first recourse is supposed to be calling the police (who would have come to his house and advised the searchers). He could speak to the searchers and warn them away.
He can't use a weapon, as others have said, to defend his property line - only his house. Now, he didn't actually use the weapon, but if his intention was to use it as a warning, it's in a gray area. Naturally, LE isn't going to go after a potential murder suspect for a minor thing like "brandishing" and the very word "brandishing" is something juries usually decide (although...brandishers usually give a plea to a lesser charge).
Because brandishing (which is, I believe, still on the books in Colorado) is a problem, Colorado has another law about "menacing":
Consequences of menacing with a deadly weapon | The Foley Law Firm
Brandishing implies that the gun is "out and waved around," menacing includes verbal threats or
showing a weapon. Intent needs to be shown (the menacer had to mean to scare people - and so far, nothing I've read indicates that BM did this for any other reason than to scare people off).
In fact, you seem to think his goal was to scare people off his land by showing a weapon. It would be a minor charge (a misdemeanor, most likely), but the person who was scared would have to complain to police directly about it.
Perhaps BM is operating under his views of Indiana laws, but I can tell you that in Colorado, as well as many other states out West, you cannot do this legally. It is illegal to walk around with a weapon in some states, even in your own yard (and in CO and CA, "weapon" in this case includes bows and arrows, and in CO, if the person is "menacing," it can include other common objects like baseball bats, hammers, power tools, etc.
