How can Krypton be a noble/inert gas if it reacts with fluorine?

Yeah, that's ridiculous.  The other thing is that it makes Superman weak.

NO! Noble gasses are difficult to react but not impossible. All of the noble gasses have been combined in the laboratory with Fluorine because it is the most reactive element known. Most of these compounds are explosive, which means they are very tenuously held together and eager to disassociate back to their elemental forms.

Group 18 of the Periodic Table was named as the inert gas group due to its first few inert members such as He, Ne and Ar. These elements have highly stable valence shell configurations in their s and p-sub shells and are therefore resistant to ionisation by other more reactive elements.
Its later members like Kr are also inert gases under normal conditions, but the presence of a d-sub shell gives them a chance to expand their valence shell octet. This crucial expansion allows them to share electron pairs with elements of high electronegativity like F and hence form compounds.