Although I am admittedly not familiar with the low-level details of their design the way that AT was designed does allow for parallelisation of AT processing as communication between ATs does not occur "whilst they are processed" (any token amounts or messages sent from one to another don't apply at the point that they are executed but effectively after all the ATs have been processed for that block).

It may be that the Ethereum design doesn't work the same way but for sure you can design "smart contracts" that allow for parallel execution (you simply cannot have the state of one affect any other within the bounds a block).