Almost any weapon would have done damage smuggled into that situation.
An overlooked part of this. A mortar smuggled in the same way for instance would work fantastically as well, the primary difference is that drones are easier to operate over wireless connections and more precise while being more expensive and with far less firepower for the weight and volume. Now using a drone as a forward observer, possibly even fitted with a laser designator for guided mortar shells, with an automated remote control mortar system concealed in a vehicle or building, that would be the best way to launch such an attack in regards to maximum firepower on target. The firing report could certainly be an issue compared to the far more silent launch and possible jinking path of a drone, but this can be mitigated to an extent depending on the situation, and it's a lot harder to shoot down incoming mortar shells than drones.
Now that all assumes you have either operators nearby or a wireless connection, on the other hand rocket launchers have been concealed and used for surprise attack for decades, see the cool rocket launcher trucks used by various totally radical Muslim groups, and of course the Viet Cong and their usage of rockets against many targets far better protected than these sloppily defended Ruskie bases.
A bunch of old rockets launched from a modified container could be accomplished remotely with nothing more than a time activated firing circuit, no local operators or wireless links required whatsoever, just park it in a pre-calculated location and launch. These attacks have always been possible, and have always been done, there is just newer, different technology being used to accomplish them.
The peak of innovation: Arabs blowing up other Arabs for Arab reasons
Reality is, in a serious total war, which this retarded slav circus is not, all the civilian long range wireless systems are absolutely being turned off and civilian traffic is significantly restricted so attacks like this become much more difficult. Infiltration operations for instance in a hot war between the USA and China probably won't look like this one, that'd be far more conventional commando operations, with the occaisional fifth columnist attacks and industrial sabotages until the internment camps get filled up again.