What is the difference between git svn init + fetch and git svn clone?
They are the same. Git has many high-level commands which call other more low-level commands. The high-level commands are oriented towards the user to make everyday tasks easy and the low-level commands do a single thing in terms of git's data structures. In git terminology they are called porcelain and plumbing commands.
Cloning is what a user wants to do and this includes initing a repository and fetching commits from an SVN repository which are totally different repository tasks.