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.
Of didn't say that they were buying FTX, but crypto companies. There's no talk in the articles that they would invest in cryptocurrencies but cryptocurrency companies, and those are down because of the case FTX .
And why they do this? They see that they can get them almost free and they are in it for the long term. Many crypto companies also might have better fundamentals then most cryptos and actual value behind them. Even if they are now having hard times.
As what I read also he is interested investing on crypto companies since maybe he can buy them at the cheapest price which maybe Sachs can take advantage with especially if the company has a coin/token which have a good record before and have huge community since for sure for what action he do he can hype it up and maybe he can also benefit if the coin/token price will pump up after the hype he bring after they bought the companies.
Hype can be converted to money so that's a smart move for him to take risk with.