If BG did say "They're down the hill", it could indicate that at least one of girls was planning on meeting somebody. The more I think about it, the more I feels this was the case:
One of the victims intended to meet either an individual, or a group of people. BG suddenly and confidently strides out onto the bridge. The girls find him unsettling- he is not whom they were expecting. They take his photo. But.... BG is not so unsettling that they refuse his order. They go down the hill with the expectation that the others in the meeting are down there.
In contrast, had BG been a randomly encountered person with a weapon, I think the girls would have acted surprised, and given the automatic response of "What?", "Why?", "What are you doing?", "Don't hurt us".
Likewise, if BG had pulled a "I am a security guard, you are trespassing..." ruse, there would have been some other discussion as well ("We come here all the time", "I thought this was part of the park" etc.")
In the end, both victims were very familiar with the area and had probably never seen railroad crews there, let alone a non uniformed "security guard" enforcing trespassing. Likewise, the victims may have been vaguely aware that the bridge was to be part of a public park- thus an even lower chance of trespassing enforcement.
As a result, BG as a "security" guard may well have triggered more alarm bells than the slightly creepy BG telling them about a meeting that they expected.