What's more wild is that $20k price was what it was in part because FTX intentionally attempted to crash the Bitcoin price to get out from bitcoin denominated obligations, and because the bankruptcy fixed prices at a time before they had fully recovered from FTX's actions their manipulation was successful.
You’re getting the dollar value of your investment when the bankruptcy happened, which is the only legal currency of the country where it was headquartered, so you are getting 100% in that sense. Maybe in Ecuador where Bitcoin is legal tender it would be different.
This has nothing to do with legal tender and everything to do with the bankruptcy code. Part of the point of bankruptcy is avoiding a race to submit claims. If you use market value, then someone who was paid out earlier or later could feel aggrieved. When FTX went bankrupt, the clock was stopped and the value of every asset frozen. We ringfence some things, and there is a decent argument for ringfencing crypto held in custody, but FTX wasn't doing that, it held some crypto, a bunch of crap and a spreadsheet promising Bitcoins to randos.