Of course, given the depth of financial depravity on display here and the incredible thinness of the financial defense, one must also entertain the theory that they did do the due diligence, they were aware of how dangerous this was, and they invested anyhow for other reasons. And that those reasons are probably not good.
There have been many cases where companies go to great lengths to do accounting alchemy to fool even very smart auditors, but we are clearly not dealing with that here. Binance or whoever was offering to buy FTX out literally figured out within single-digit hours of looking at FTX's books that they were not interested. A person skilled in the art can literally glance at these books and figure out that they are worthless. I do not believe there is any credible theory that any investor could possibly have been unaware of these issues.
FTX's books don't so much have red flags as that they are printed on red flags, with ink derived from red flags, a custom-made red cover made out of more red flags, and each page, when opened, has pop-up red flags along with a little electronic speaker that plays Red Flag by Antigoni while you get sprayed with Red Flag perfume [1].