I think he compared it to the Alexa rankings which can provide a general estimate how many unique visitors the site can get.
However when it comes to gambling it doesn't mean anything.
300k per month means 10k per day or 400 per hour. There's no way Rollin had anything close to 7 unique visitors per minute. Or maybe they somehow managed to count faucet farmers who use many unique IP-addresses in the industrial scale farming they did.