It isn't necessarily that Sequoia (or any of the other VCs) had dirty money to launder. It could have also been that they wanted to use clean money for unclean purposes.
To be clear: I'm just letting my mind wander in response to your question, not saying that's what happened.
It could be simple greed by incredibly simple-minded VCs or it could be something worse. Somebody asked what the something worse could be, and I replied.
We are learning more and more about all of this each day. There was massive fraud going on within FTX, and I don't think it's terribly far-fetched to believe that it wasn't contained within the company.