It may seem to you. I asked real students on campus, who had no idea what 4/16 was. Yet a student has lost their educational opportunities here, and likely life ruined.
I'm betting a lot of people don't know the date of the Boston Marathon bombing. But threats are meant for people who DO recognize the significance, and the people who watch for threats do know what these dates are. The key point here is: did what he post actually look like it might be a threat? I say yes, and the fact that the people you asked didn't know the date doesn't have any effect on the situation.