I didn't catch that, thanks for point it out. Like another commenter said: this seems to be an in-place upgrade which is not advisable for major PostgreSQL versions. Additionally, it looks like this involves restarting the cluster (downtime). The Database Migration Service looks like it wouldn't require downtime.
Folks in the know claim that in-place major version upgrades are unadvisable, as it's not a super heavily tested area of postgres development and in the worst case scenario you might end up with unwanted side-effects in the data that can go uncaught for a while.
When 9.5 lands in RDS, they will inevitably take six months to roll out the major version upgrade to 9.5 feature, during which time this can be very useful.
