Welcome to the forum,
To start with, I like how they have security, the fact of placing the words or phrase to save the ceunta seems excellent to me, also the fact of requiring 2FA, I also did some tests and verified that according to the pools of insanity, The 500xBit.com platform does comply and does not manipulate the market movements in its favor, which speaks highly of the honesty of the site, and this is something that is far from scams. It would be excellent if you considered that they could take other options of alts, at least the most fashionable ones, DOT, UNI, BNB, ADA.
Is it legit will you get the bitcoin though if you win?
I think it is legit, you can try it by yourself.
After all, it is gambling which you can win or lose so you need to be careful.
I think they will add the other altcoins in the future after they see the potential to make more income from the bet.
They need to focus on running their business first and they can add it later after the site can run properly.
I don't think they will ruin their reputation this early, if they will not let you withdraw what you should have. But yes, adding other alts may be a good one to attract more players. So that means, we may not only see betting on btc price but also other alts, which for me, is interesting. They should run some rewards program or contests to entice these gamblers to play on their betting exchange.