This seems like a frivolous lawsuit to me. I would argue that the exchange/wallet provider is more at fault here (if applicable) for using poor security. SIM hacking/SMS intercepting is something that should be addressed, but these issues have been known for quite some time. Companies shouldn't be relying on them as authentication mechanisms. Seems like he is grasping at straws here since he can't sue BYOB (Be Your Own Bank).

edit:

I found the actual text of the lawsuit here, and it's hilarious.

https://www.law.com/therecorder/2018/08/15/prominent-crypto-...

The hackers also used the phone to hijack Mr. Terpin’s Skype account to impersonate him. By that means, the hackers convinced a client of Mr. Terpin to send them cryptocurrency and diverted a payment due to Mr. Terpin to themselves.

Yep, AT&T totally at fault here!