AT&T's data was compromised as one of Snowflake's many customer breaches (Ticketmaster/LiveNation, LendingTree, Advance Auto Parts, Santander Bank, AT&T, probably others [0][1]), which occurred and were notified in 4/2024 (EDIT: some reports says as far back as 10/2023). Supposedly these happened because Snowflake made it impossible to mandate MFA; some customers had credentials stolen by info-stealing malware or obtained from previous data breaches. Snowflake called it a “targeted campaign directed at users with single-factor authentication”. The Mandiant report tried to blame unnamed Snowflake employee (solutions engineer) for exposing their credentials.
How much responsibility Snowflake had, vs its clients, is not clear (for example, seems they only notified all other customers May 23, not immediately when they suspected the first compromise). Reducing the analysis to pure "victims" and "criminals" is not accurate. When you say "criminally prosecute those whose negligence made this possible", it wouldn't make sense to prosecute all of Snowflake's clients but not Snowflake too. Or only the cybercriminals but not Snowflake or its clients.
[0]: The Ticketmaster Data Breach May Be Just the Beginning (wired.com) https://news.ycombinator.com/item?id=40553163
[1]: 6/24 Snowflake breach snowballs as more victims, perps, come forward (theregister.com) https://news.ycombinator.com/item?id=40780064