This only indicates that an AI coding agent was used to write an exploit.
No such circumstantial evidence can prove that an AI model has been used to find the bug.
Of course, it is quite likely that an AI model was used to speed up the search for bugs, but this can never be proven as long as you see only the code used to exploit the bug.
They don't say "proven", they say "we have high confidence that the actor likely leveraged an A.I". Do you find that assert too different from your "it is quite likely that an AI model was used to speed up the search for bugs"?
No such circumstantial evidence can prove that an AI model has been used to find the bug.
Of course, it is quite likely that an AI model was used to speed up the search for bugs, but this can never be proven as long as you see only the code used to exploit the bug.