It would help to learn if they would give the English translation the same pattern as the original. Why not 'You are the one of whom I think', We know that's not currently common parlance, but we know our own lingo & could easily take the direct parallel and make it casual. When I was learning it, the whole piace mess would have been infinitely simpler if the translations had used the phrase 'it wasn't pleasing to me'; we would then automatically say to ourselves. 'I Don't Like It'
Is that the distinction they want to make? Between simple present and present progressive? Okay. I get that. In most cases, I would argue that the two are interchangeable. In this case, I can see that they might not be (i.e., if the simple present implies a general status -- "you are the only one of whom I [ever] think", and the present progressive implies a temporary or momentary status -- "you are the only one of whom I am thinking [when I do x, or today]"