This is not true in the general case. E.g. setting a field to true from potentially multiple threads can be a completely meaningful operation e.g. if you only care about if ANY of the threads have finished execution.

It depends on the platform though (e.g. in Java it is guaranteed that there is no tearing [1]).

[1] In OpenJDK. The JVM spec itself only guarantees it for 32-bit primitives and references, but given that 64-bit CPUs can cheaply/freely write a 64-bit value atomically, that's how it's implemented.