From my observations, yes, it is sequential. From the docs:
Transformers act as collection of INVOKE steps embedded in a single MAP step. However, transformers in a MAP step are independent of each other, do not execute in a specific order, and might not execute in the same order each time the MAP step runs.Consequently, the output of one transformer cannot be used as the input to another transformer
There was a thread many moons ago that covered this briefly. In that thread it has a quote from older docs:
When inserting transformers, assume that webMethods Integration Server concurrently executes the transformers at run time.
This is undoubtedly the source of the apparent misconception and is no longer in the docs. I have not come across any evidence that indicates transformers run concurrently. If any one has such evidence I would be very appreciative if it can be shared.
I too use published docs (sometimes locally) to introduce parallelism or async activity when needed.