I think the safest option compared to trading and all that is the fact that as long as there is no hacking involved you are bound to make money. If you are considering bitcoin as a whole for investment and decide not to invest into bitcoin then yeah you can invest into something "safer" yourself.
So, the "safe" here is not for the people who are holding fiat and considering what to invest into, the "safe" here is the people who have bitcoin and don't know what to do. If you have bitcoin at your hand and you are going to invest into something eventually its safer than trading because in trading you may lose money if you buy high and sell low but in casinos you invest and you keep making profits after profits which increases your bitcoin holding in the long run.
Trade and investment are both at risk and not just trade that can suffer losses due to wrong buying and selling decisions. Investments also sometimes suffer losses when market prices decline. We will not experience profits and losses if we do not carry out trading or investment activities.
But, at least if we make a long-term investment, especially investing in a trusted gambling site, the money or bitcoin that we invest is managed in such a way by the developers of the gambling site for our benefit.