There's a way to get an extra imbued heart from a super frenzy, such that you can upgrade a proto weapon immediately after the first imbue. This persists such that you can enter super frenzy 2 with an already level 11 weapon, super frenzy 3 with a level 12 weapon, etc.
What I think happened...
A friend joined us for a successful super frenzy treek run before she even had the level 4 treek and owlitzer heartstones. This meant that she missed out on the ability to unlock proto weapons (though perhaps the game tracked it as the obligatory first victory, anyway?) We later unlocked the necessary level 4 heartstones so she could unlock super frenzy, and then we beat super frenzy treek, which gave her the imbued heartstone. She was able to unlock proto weapons at this point, and crafted a proto bow. We beat super frenzy treek once again, and her bow became empowered, but also received another imbued heartstone. At this point, the blacksmith offered to convert her proto bow into the treek bow, but she still had an extra imbued heartstone, which allowed her to immediately upgrade it to level 11.