FTX seems to have its own stable coin named “USD” that groups all stable coin except Tether (USDT).
It looks like Koinly consider this FTX USD stablecoin vault as FIAT USD and then calculates profit.
At least, it’s what I understood from here:
“USD, USDC, TUSD, PAX, BUSD, and HUSD all count as “USD Stablecoin” balances. Depositing any of those to your FTX wallet will credit you 1:1 with USD Stablecoins.”