More recent discussion on this in the HK Moms and HK Schooling groups on Facebook indicate while it is "legal" there is a VERY important caveat to that statement.
"It is legal, with government approval" or perhaps more correctly, "it is not illegal but not encouraged".
The attitude of the HK government is that every kid deserves the best education they can possibly get, and for the vast overwhelming majority of kids the "best education" they can get, in their view, is a traditional school-based system.
The EDB holds that school education provides children with a broader and more structured formal curriculum as well as rich learning experiences, and creates an environment where students can interact and exchange ideas among peers and with teachers. All these are essential for the all-round development of children in the areas of ethics, intellect, physique, social skills and aesthetics. Hence, the EDB does not encourage home-schooling in place of formal school education.
LCQ11: Home-schooling
If you dig into this Q&A from a few years ago, you'll see that homeschooling is approved by the government on a "Case-by-Case" basis. As of the timing of that query, there were about 25 approved home-schoolers in Hong Kong. Anecdotally one was relayed to me as an expat couple on an outer-island who spoke French and couldn't logistically get to FIS for schooling so they were allowed to homeschool, or something like that.
Now, that there are 25 approved cases (or maybe even double or triple that number) doesn't mean that's the limit--there are plenty of people flying "under the radar" and doing it anyway. However, if a nosy neighbor decides to report a family to EDB for not sending their kids to school an investigation can be launched and home visits from EDB and maybe even an attendance order (i.e. demanding the kid go to a regular school) issued. There haven't been many cases of this, but there have been a few.
Basically, you can homeschool, but the government may get involved. They encourage you to register (but you don't have to) and they reserve the right to visit and check on the welfare of the kid, along with encouraging you to go a more traditional route. If they feel you aren't doing a good enough job, they can issue an attendance order and force you to attend a 'regular' school.
There is a homeschool FB group and a homeschool website for more info.
Home Schooling