That shouldn’t be the case, according to the git-config man page on pull.ff:
(…) When set to only, only such fast-forward merges are allowed (equivalent to giving the –ff-only option from the command line). This setting overrides merge.ff when pulling.
The configuration pull.ff has been introduced in Git 2.x, so it won’t work as expected on Git 1.x — it will probably pick up the merge.ff configuration and use that when doing a Pull.