I am not sure about fixedfloat, but binance allows account freezing if you can give them an official investigation letter from your local police and you can connect the dots [they provide a table for this] that prove the address used by the scammer is one of theirs. Maybe he's referring to this authority, your local police.

IIRC Fixedfloat claim they do too. But since the trade has most likely been completed and money has already left their end, they will just add the address/es involved in their blocklist then freeze funds if the perp transacted with the same address.

OP could try to request the perp's trade information as well but as always, fixedfloat will most likely request legal paper/s from op's local police too.