You linked Microsoft's investigation report on the exploit.
The attackers first managed to get access to Microsoft's development network, noticed a crashdump, understood the possible significance of that, dug through it, found a private key, then acquired enough insight into Microsofts authentication systems to understand how this key could be used beyond its intended purpose and then executed on that.
And you don't believe they left persistent backdoors in some high-profile targets?
The conclusions being drawn are … entirely appropriate. Your argument maaaaaybe makes some sense applied to general public random cloud customers. Backdooring indiscriminately just increases the risk of discovery. But large companies and government users? You have to assume compromise, anything else is incredulously naïve.
cf.:
https://www.microsoft.com/en-us/security/blog/2023/07/14/ana...
> Storm-0558 operates with a high degree of technical tradecraft and operational security. The actors are keenly aware of the target’s environment, logging policies, authentication requirements, policies, and procedures. Storm-0558’s tooling and reconnaissance activity suggests the actor is technically adept, well resourced, and has an in-depth understanding of many authentication techniques and applications.