Earlier quoted context omitted.
Only the 5.8bn in FTT is being claimed to be worthless. Not the full 14.6bn in assets. If that's true (and the article itself lists reasons it isn't true!?), that still leaves 8.8bn in other assets, vs 8.0bn in debts. The article itself says the same when it says 88% of equity. Equity is assets less debts. So as long as they have any equity left at all, they're not insolvent.
You assume that during a firesale they would actually get value approaching what they claim is worth 14.6B. Since FTX and Alamaeda are by far the largest holders of FTT and significant holders of Solana they will destroy the market for both if they had to sell. It's likely the real value of their assets is far lower, as the other commenter suggested.
If you assume a fire-sale on everything, then everyone is insolvent all the time...