The word "It" can be used in two ways: 1, as a pronoun in place of a noun; 2, as a generic pronoun for some event. In the first usage, "It surprises me a lot" may mean "Your progress surprises me a lot" or "Your piece of work surprises me a lot". Some examples of the second usage. It surprises me to learn that she is a mother of three. (That she is a mother of three surprises me.) It wouldn't surprise me if they got married soon. (The likelihood of their getting married would not surprise me.) It surprises me that you have no formal training in singing. (The fact that you have no formal training in singing surprises me.) From the context of the passage, which is talking about the progress the contributor had demonstrated, "It surprises me a lot" can refer to both the progress and the work that illustrates the progress. The sentence may mean It surprises me a lot to learn of your tremendous progress. or It surprises me that you have made tremendous progress. or Your piece of writing surprises me a lot. or Your progress surprises me a lot. Given the ambiguity, it is up to the readers to interpret the writer's underlying idea.
The second usage of "It" is as a generic subject for some event.