500k unique players is exactly what they mean by 500k monthly users/monthly active players. It doesn't mean 16k players logging in 30 times a month.
So Elite Dangerous has 500k unique accounts playing in a month, Star Citizen has 750k unique accounts playing in a year. We don't know for sure how that compares but it certainly looks like Elite has a healthier population, especially when you work out how SC's playerbase play on average less than an hour a week.