If you're referring to the email that said
"Embassy Tripoli reports the group claimed responsibility on Facebook and Twitter and has called for an attack on Embassy Tripoli.", please note that those claims did not originate from Ansar al-Sharia's official channels. And in fact Ansar al-Sharia denied responsibilty as an organization - though they did leave open the possible involment of it's members and referred to the attack as "spontaneous popular uprising".
To better understand the significance of that you'd have to understand what kind of a militia group Ansar al-Sharia is. As a loose organization some of its members may do something on their own account without any central planning or even approval (bit like how Anonymous works).
Therefore the reports of Ansar al-Sharia trucks and members being present during the attack do indicate that its members were involved but do not prove that Ansar al-Sharia was responsible or that it was a planned attact by Ansar al-Sharia.
Besides, there were also other militia men and groups present, for example Martyrs of 17 February Brigade, and from early conflicting reports and accounts it was not clear who were actually involved in the attack as some militia men were attacking and some were defending the compound.
Apparently Martyrs of 17 February Brigade were defending the compound against the attackers, though Greg Hicks opionates that they were actually complicits in the attack.
So? "Surrounded" implies a siege. It was not. They were actually asked to be there by the hospital, as Benghazi's Al Jala Hospital had invited them to provide security. At the time Ansar al-Sharia was one of the many militia groups providing security in Benghazi including the hospital.
But because of their ideology they did not enjoy much popular support. Ten days after the attack, when the involment of Ansar al-Sharia members had become apparent, angry residents of Benghazi stormed its compounds and bases in a protest and drove Ansar al-Sharia out of the city among with some other militia groups.
Since February Ansar al-Sharia has been returning to Benghazi to provide security under the pretext of being nice with outreach programmes and all, but not everybody is buying it:
Ansar al-Sharia returns to Benghazi.
At the time there were lots of conflicting reports and accounts of the events, the privilege of hindsight makes it possible to pick and choose the parts that seem to fit. Remember how according to early reports ambassador Stevens' body was dragged in the streets and violently assaulted by an angry mob? Yet in reality nothing such took place and actually people rescued him and took him to the hospital.
It is neccessary to have substantial
vetted intelligence before providing a hypothesis or establising a conclusion. Conflicting information does not allow that and more is needed. That takes time. Hence the preliminary nature of information was emphasised early on.
