You might be very surprised at what Physicians are trained and educated to do in emergencies.
The doctors associated with this event and those who responded are not just student doctors. The AMAA is a big sponsor in this event and the DMAT doctors are not newbies who have no clue about trauma. Responses are also going to be different depending on the area and resources. Have a city like Boston with several respected hospitals and trauma centers will make it easier. These are professional teams who are well prepared and don't enter situations "mcquyvering things. You can refer to another discussion here for Paramedic "mcguyvering".
With the AMAA, there were several doctors running and several more supporting them on the sidelines. Those managing these events have learned lessons from previous incidents which have included everything from cardiac arrests to shootings.
Were there any EMS associations which actively encouraged or supported their members in the Boston Marathon?