In my understanding of this answer, the reason for person A to develop learned helplessness while person B to do not mostly (if not totally) boils down to the explanation styles of the two. But then would that mean that learned helplessness can develop without the subject being conditioned helpless? If I tell my friend that "It's impossible for you to solve a math problem, so don't bother trying it", then would my friend develop learned helplessness, even when he haven't known what math is?
I think what you are asking pertains to how learned helplessness is defined. If sticking to the traditional behavioral definition, then yes, 'learned helpless' is a response to an accumulation of stimulus-response events. See (Lerner, et al. 2018)
However, if you depart from the traditional behavioral definition, you can incorporate the individuals role in contributing to the learned helplessness. Moreover, learned helplessness is no longer the result of being a passive recipient to stimulus-response events. So in this case, your friends learned helplessness of math is not only dependent on you (includes the role of the individual, like mood, previous math experience, etc.), and is not dependent on the amount of times it was reinforced (that the math problem is hard, you can't do it).
Departing for the traditional behavioral definition, the newer approach is in line with developmental systems approaches that prioritize the role of the individual. See Sameroff (2009).
Lerner, R. M., Lewin-Bizan, S., & Warren, A. E. A. (2011). Concepts and theories of human development. In M. H. Bornstein & M. E. Lamb (Eds.), Developmental science: An advanced textbook (pp. 3–49). Psychology Press. https://psycnet.apa.org/record/2010-22284-001
Sameroff, A. (2009). The transactional model. In A. Sameroff (Ed.), The transactional model of development: How children and contexts shape each other (pp. 3–21). American Psychological Association. https://doi.org/10.1037/11877-001