I have friends who have asked UNSW staff about this and the reasoning behind it was differences between SDD and tertiary Soft Eng/CS and that it's possible to go well in SDD but not actually know how to program well (or at all really) as the syllabus gives teachers free will for the practical components essentially.
To be honest, in my opinion, it is bullshit because HSC Physics did not prepare me in the slightest for university physics and the two were nothing alike. It's also ironic that UNSW CSE (CS and SE) courses do not have compulsory physics.
Physics is a proper subject that helps develop the intellectual capabilities of a person at a much higher standard than SDD which is a much more iffy subject
HSC Physics is a whole bunch of wafty bullshit you cram on the history of physics for the most part, it's a heavily watered down course which was intentionally done for mass consumption by students regardless of their ability or chosen level of maths.
SDD has a large portion of theory which was becoming obsolete when the course was made and even more so now (agile development is the norm in the industry and the waterfall model is relegated to textbooks). But despite that SDD offers some students a first opportunity to formally learn programming, IF you have a good teacher. It also possibly provides you with 3-4 programming assignments over year 11/12 - programming actual stuff is the best way to learn and it can be very hard to come up with project ideas so the assignments can be great for that.
The algorithm component of the course also made you think and imo helps develop logic and the ability to actually think - you CANT cram for those quesions and either have to have the "knack" or put in practice to be able to tackle the questions effectively. Even though that is just my opinion it is one of the reasons there is a push by some people to make learning programming compulsory for all students.