No it is not.
Homeschooling in Hong Kong is not against the law. This has been confirmed by the previous Permanent Secretary of the Education Bureau, Ms. Cherry Tse Ling Kit Ching and also raised by Legislator Dennis Kwok at a meeting of the Legislative Council. The EDB treats homeschooling on a case by case basis. *
http://www.homeschool.hk/#/hkLawsAboutHomeschool
I stand corrected, homeschooling is sometimes permitted BUT you have to convince the EDB that you fulfil their requirements. I still think it's dangerous to blindly assume that homeschooling is allowed without restriction. That could lead parents into potential hot water.