Let's say for sake of argument the investment by FTX was made at a valuation of $1B and Anthropic is now worth $10B.

Could Anthropic now say, "Oh no, that money was stolen, the investment was never valid, we're returning every last cent." and essentially do a huge share buyback at a steep discount to current valuation?

I guess that's not how corporate liquidation work.

A liquidator will evaluate the assets current market value and try to liquidate them in the most advantageous way for creditors.

Which means: they'll try to get at least the valuation that Anthropic got from the AWS deal. The problem is: they need to find someone willing to buy these shares at this price. If they do so, FTX creditors will be happy.

FTX was an investment company. Of all the fraud that company did do, this is like the one thing they were supposed to be doing.

And it's up, so if you think they should renege investments, start with the ones that failed and not the profitable ones.

They attempt to sell the shares of the company it invested in.

As a very crude analogy, if someone got access to my brokerage account and used to money in that to buy a Ferrari and got caught, do I get my money back or get the Ferrari (and presumably have to sell it)?