This is different from imaginary value inflating and disappearing (i.e. tokens that never had liquidity - where mark to market never made sense, etc). In that case the whole thing is a mirage.
But if $8 billion dollars went into FTX, that money does not just "disappear" from depreciation - someone else's (or many people's) purchasing power increased in relative terms proportional to the losses suffered by FTX's creditors/depositors.