It's not RS → F because that's just not what the sentence is saying.

You have to take the Only If grammatically literal--Plumbers can fly, ONLY IF they wear a raccoon suit.

The way "Only If" is presented indicates a necessitated restriction--it's similar to "on the one and only condition that". This is entirely different than what "If" would indicate.

If there's a flying plumber, it means he must be wearing a raccoon suit because the raccoon suit is the ONLY way that a plumber can fly.

Imagine we translate the sentence as RS → F:

Just because you see a plumber wearing a raccoon suit, doesn't mean he can fly. Imagine you see your plumber wearing a raccoon suit for Halloween. Can he fly? No.

Now, imagine we're in Q1.1's universe and we finally see a real flying plumber!

Based on Q1.1's laws, I know that in order for him to be flying, he must be wearing a raccoon suit.

Another short example--imagine in our universe:

I can breathe only if there's oxygen.

If there's oxygen present, does that require me--wherever I am in the world--to 100 percent, absolutely, without a doubt be breathing? No. What if I'm dead? What if I'm holding my breath underwater?

But, if I am breathing, then we know with absolute, 100 percent certainty, that there is oxygen present because that's the only way breathing is possible.

A → B

X → A

X → A → B

-----

X → B

You can also think of it as:

If A, then B

x is a part of A

Therefore, x must also be a part of B

A → B:

- If I am at Lake Minnetonka (A), then I am in Minnesota (B)

xA:

- I was purified (x) in the waters of Lake Minnetonka (A)

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xB:

- Therefore, I was purified in Minnesota

Also, x has A, not the other way around (same with B)