additional context from the article regarding that particular statement:

"[the statement] was oversimplified... In reality, the tests involved “red teams” of N.S.A. analysts who were using Mythos in a highly tailored environment that would be extremely unlikely for an adversary to replicate, officials said. The red teams began their tests within classified N.S.A. systems designed to be accessible only from certain computers and completely cut off from the broader internet.

The tests found that Mythos was able to identify cybersecurity flaws within that classified network quickly, but it did not actually break into those systems, the officials said."