It's strange that this forex broker would allow arbitrage. Normally they don't. And from what the OP is describing, it looks like some kind of latency arbitrage which usually results in complete forfeiture of all profits made.
Yeah, it is latency arbitrage. Nothing illegal about that, brokers just don't like it. If it's not prohibited in their terms, only thing they can do about it, is to introduce slippage.
Anyway this specific broker not allows it, they even encourage it. When going to their website, the first thing You see, is a popup telling about arbitrage.