Beware, it is a fully SCAM exchange.
I just did search for a new exchange with futures trading and tested this one. First - they lies about his trade volume. Actually the order book is empty, I can't trade.
But when I tried to withdraw my funds (over $100k) I got error "Withdraw forbidden". Later I received the letter that they block my funds due to "anti-money laundering risk". I checked my bitcoin account with AML bot - it is Low Risk. They asked me for documents from where I received my funds.
I'm in bitcoin from 2012 (registered here from 2013 as you can see), most of my bitcoins doesn't have "documents". Most of exchanges I used already doesn't exists. But my bitcoins are clean.
Providing the screenshots of the letters [I'll assume you're referring to an email chain] might help add more context and understanding here. With the current description you gave, is it possible that they're asking for Source of Wealth instead of the origins of the 100k USDT back to the very beginning? Like your employment slip, or some inheritances, whatever documents to prove the source of your wealth. Or, if they indeed asked for the origins of the funds in sense of the source of deposits, perhaps they're not requiring you to give a full detail of entire transaction back to a decade ago? Perhaps the last two or three transaction chains?
Their 24-H volume for spot trading
according to CMC is USD 1,127,413,542.60, and though you've said that their futures trading volume is fake, thus that number is probably also fake,
the small numbers of raised complaints here [I just did a quick keyword search and glanced at the numbers, haven't look at each of the threads to see what's they're about] and that they've been around for several years tends to give them --big or small-- benefits of doubt that it might be a case of miscommunication. Thus, repeating what I said earlier, giving more context will likely helps us understand the situation better.