You must have misunderstood the news article. It didn't say they will buy what's left of FTX but they will buy or invest in cryptocurrencies because the prices are down as a result of the FTX implosion. These banks are business opportunists at the end of the day and will take a calculated risk if a chance presents itself.
That's right. They risk with the investment in Bitcoin and speculate just like everyone else on profits. They do the same with the "normal" currencies. With cryptocurrencies, the risk is only much higher. You can also call it an additional asset class. Investing in currencies, bitcoin, gold, real estate, cars and more is just a way to invest your money and not let it destroy by inflation. If possible with the smallest risk.