No - the embassy was empty - nobody was hurt...
The embassy grounds are soverign US territory. The fact that nobody was injured (purely because the rioters failed in part of their objective) is irrelevant — this is every bit as much an act of war as 9-11 or Pearl Harbor.
It was an act of war. There are many unresolved acts of war around the world including a similar rampage in Tehran.
They attack US Soil in a technical sense, this is an act of war. Unless you are saying it is an abandoned embassy