It is possible that doing both serial and on-depends would create a conflict, i.e. that the acyclic graph defined by the on-depends wouldn't be compatible with the ordering given by sequence of the components. I doubt, but who knows, if asdf actually checks for that.
So sure, that's redundant. But yeah, redundant isn't necessarily a bad thing. Stating the on-depends in addition to the serial might provide a bit-o-info for the reader about the structure of the code.
And the developer might occasionally comment out the serial to gain the benefits that follow from that.