It seems like a self defense mechanism of hers. She wants the position and reacted that way to defend herself/her claim to the manager position. If she was your friend and didn’t work at the store, her reply most likely would’ve been supportive. Just my two cents. Go ahead and apply - it’s a good sign that you were encouraged to apply by your managers! Edit: I’ve only encountered having my stutter being used against me by people who’ve been self defensive or genuinely wanted to hurt my feelings. Both which are valid reasons for me to discard their comments as irrelevant.