You are quite right. Normally, the HD would incorporate both paths. In this question, they are trying to illustrate the transition from H->D only (because you need to break each path up so you can work out the answer). This is not very clear labelling and in order to understand what they are doing, you need to look at the workings of their expression to see that they are only referring to the single case from H->D without going through S.
I am not sure there is a clearer way to label this expression so, in the future, look out for what they mean when they say HD and be clear yourself (often they will ask you to explicitly state what you mean in the start of the question and even if they don't, do it anyway so that your answers are not misunderstood and it's clear what you mean...if that means being better than the way the memo wrote it, then do so an be proud at how much clearer your answers are to them!)
A side note, it's important to understand that the memo's are not written as notes for students to learn. They are written to help markers mark. This distinction is important to understand as you can write answers better than the memo. Don't think that the memo is the gold standard or necessarily the best answer, but it is an answer which often holds a lot of merit and understanding where and why it has merit (and where it could be better) is part of the learning process so it's very good that you are asking these kinds of questions!