The same faq says that: "The system (via the Community user) will automatically protect a question that's had three answers from low-rep users deleted or 5 answers from low-rep users posted in the past 24 hours."
So perhaps the system does it after 24 hours even if nobody does anything to the question.
Anyway, it is not surprising that questions shown in HNQ which is additionally open to interpretation attracts some answerers from other sites.
To be honest, I am not sure to which extent I should consider this a mathematical question.