[Dutch Alex]

Although I mostly ask feedback from you all over true situations I have experienced and do my own homework (as in the rule test every year), I'm puzzled by this T/F-question:

Situation:
Batter hits a line drive that, after being deflected by F4, hits the base umpire standing behind the fielder and then caught on the fly by F9. Is that a legal catch?

The situation itself is kinda strange/not real to me.
Covering the ISF ruleset, I've found the rules 8.1.e / 8.5.f1 and 9.2.d-e stating that it's a fair batted ball. By rule 8.2.b it's not a catch.
So my question to you is; what is it?
It's not a legal catch, but a fair batted ball. How do we treat this?
Clearly for my test the answer is "False"; not a legal catch (rule 8.2.b). However what do we as umpire's signal and how do we explain the defense team that F9 has caught the ball illegally? Does this mean that F9 has to throw the ball to 1st base on order to get an out if ball beats the batter-runner?