Of course ASMs are not necessarily sequential. In a distributed ASM, computing agents are represented in the global state. New agents can be created, and old agents can be deactivated. There could be various relations among agents and various operations on agents. The moves of different agents form a partially ordered set.
Concurrent moves cause consistent changes of the global state.
Often a formal method comes with a reasoning system. If this is your idea of a formal method then the ASM approach is not a formal method. It is system informatics where modeling is carefully separated from formal reasoning. The separation of modeling and reasoning concerns does not undermine the role of reasoning. The ASM approach is not married to any particular formal reasoning system and is open to all of them.