It appears as if one way Archegos was able to amass such a position unnoticed was ..... because he wanted to go unnoticed.
Archegos was set up as a family office, which have much much looser reporting requirements and regulations.
https://finance.yahoo.com/news/one-world-greatest-hidden-fortunes-002617417.html
And Mr. Hwang had also been caught once for insider trading already, so....... a leopard can't change it's spots?
Mr. Hwang is a private citizen and is able to do many things without regulatory scrutiny. However, his prime brokers (I'm assuming that is CS and Nomura) also failed at their jobs as soon as the losses went beyond Mr. Hwangs portfolio and went to their capital.
If it is just one such case, we are likely fine. Hundreds of such firms doing it simultaneously when the bubble has grown bigger can again bring the entire financial system down.
After this episode, it'll be a travesty if all the Basel rules are not dusted off, amped up more than was planned in 2009, and applied on the banks once again. Make banks apply stricter margin requirements and take larger P/L charge on their own book, and hold reserves for all kinds of Valuation adjustments (Capital/Funding/Credit) that have punitive additions for leverage/credit-rating/complexity and these types of shenanigans suddenly become a lot less profitable during the good times whether Mr. Hwang wants to hide the positions or not.
Someone needs to page Elizabeth Warren. She was involved when many of these rules were initially formulated.
FWIW - American banks, even after the 2017 Trump deregulation, are generally following stricter rules than European ones. Not sure about Nomura/Japan.