How is trading "safe"? There have been plenty of people that lose plenty of money and bitcoins in trading. It is not a sure ball thing that would create your great wealth. There are a lot of variables to consider when trading, and a newbie to it would not understand it at the get go. So at first for sure you'll lose some money in trading, that's part of learning.
Most likely the poster you replied to meant to say that trading is generally safe once you get the hang of it and are able to earn consistently. And this is what makes trading different from gambling where you rely on pure luck minus house edge. So no matter how long you gamble you, first, are inevitably and inexorably set to lose in the long run and, second, you won't become any better at it over time since you will depend on luck as much as before (unless you find a hole in the casino gambling engine, of course)...
In this way trading can be considered a safe way of making money
Even if you are the most experienced trader you are still not safe, well it lessen the chances of failing though. I guess that is indeed the best differentiation of gambling and trading. Gambling has a fixed chance of winning and no one can ever change that chance. Everyone will have the same chances. But in trading you can minimize chances of losing and indeed it can make it "safer."
You don't take into account a few things
Milking the casino is obviously considered as "illegal" from the casino point of view. For example, if you exploit a bug that you were lucky to find, this won't be welcomed by the casino owner, so if you don't withdraw this easy money in time, your winnings will be cancelled and you will be most likely banned (and basically the same will happen in a real, Las Vegas style casino). Milking the markets, on the other hand, can be perfectly legitimate, for example, if you find and use an arbitrage opportunity. Indeed, some ways are illegal too (at least, in most jurisdictions), for example, insider trading, but this ban is easily avoided and such trading is in fact omnipresent. My point is that, with trading, there are quite a few of sure and conditionally legit way of earning profits (unlike gambling)