I'm not 100% sure, taking a look at the code now:
https://github.com/apache/hadoop-common/blob/trunk/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/serializer/SerializationFactory.java

It looks like using the SerializationFactory let's the class pick up the
relevant configurations. Not quite as important here, I suppose, where the
underlying stuff is pretty thin, though since InputSplit is an interface it
makes it more extensible and consistent with how Hadoop allows people to
configure things. If implementing Configurable works that's definitely the
way to do it (and I may refactor things to look more how TaggedInputSplit
does)