“Not affirm” means they don’t take it as an axiom (“we don’t take this to be obviously true”)

“reject” would mean that if, given their set of axioms, it could be proven, they would try and tweak their axioms in order to make it unprovable or provably false in their system (“we think this cannot and should not be true”)

So, “not reject” means they would happily use it in their proofs, if it could be proven from their set of axioms (“interesting. That gives us a powerful tool to do proofs”)