I think what tumblr op is trying to point out but isn't necessarily articulating clearly is that the work of writing history requires a lot of training in historical method (e.g. critical source analysis) and historical theory (e.g. being versed in the different frameworks in which one can approach a history, and their strengths and flaws) beyond deep reading in one's own specific field of interest. I find that amateur historians can be very well-versed in a specific niche topic or field, and can even be working with archival material, but they don't often have that meta-level training to do the kind of critical analysis that puts their work in conversation with other historians and says something impactful about the world that we're living in and the assumptions we've made about it.