I'm an aromantic asexual who is not a woman and does not want a romantic relationship and I identify very closely with the expressed desire to 'find a Frodo'. I interpreted this as a desire for exactly what was stated: a hero-sidekick relationship.
This is anecdotal, and so not data, but it's enough to prove that this isn't ONLY about intense romantic relationships.
I have a strong desire to be a 'second-in-command' type of sidekick.
Backing up slightly, this post resonated with me very strongly. All internal fantasy, from childhood to adulthood has been about me entering the world of the hero and suborning myself to them to act as a sidekick. I didn't realize others felt differently for quite some time.
I feel the second in command style works better for me because I am attracted to heroes with slightly different preference weightings than myself. In places where the task conflicts with the heroes preferences, I could step in and relieve their distress by performing it instead. I don't know that this specifically distinguishes it from an 'assistant' however.
Going with my intuition, I would suspect that there are people who would feel drawn to the different roles. For instance I would not like to be an assistant because I'm an excellent emergency back-up leader and would be wasted in that capacity.