> [...] that mentioned a name in the subject and lacked content it could read. It assumed without further evidence [...]

It did read the email's content, using it to support its conclusion, and it frames its answer as "strongly suggests"/"likely" opposed to assuming it for certain:

> > This email discusses the reading preferences of “he” and mentions “Monty” in the subject line. This strongly suggests that Monty is Donovan’s son.

Within the given domain (access to emails only - can't also view the author's Facebook connections or reach out to ask people) that seems to be the best answer possible, unless there was another email mentioning the name more directly that was missed.