Even I'm not surprised if they say the current total users are 5 million because 3 million users was 3 years ago it's definitely different from now
Is it real? we don't know, but I can describe how an airdrop or campaign gets 1-2 million users in less than 1 month we can check here
https://coinmarketcap.com/airdrop/ this is just a simple example it might happen on a casino site.
I do not know if that number is real or not, I have seen plenty of airdrops and other things that have seen plenty of attention and seen ones that did not get any attention neither. Considering this one as getting a lot of attention, plus seeing all the advertisements they are doing everywhere, I would say that getting 2 million more people is actually quite doable.
Doesn't mean that they are all gambling a huge amount, some of them never gambled probably, just opened an account, some of them gambled only a small amount and left, some of the gambled huge amount and left, some of them gambled small amount and still doing it, so it is 0.1% or even lower that gambled a big amount and still gambling a big amount today. So, it is not really that shocking, that first 3 million is something I wouldn't know, but that extra 2 million seems reasonable enough.