Question to you: Have read the law in question? (No use arguing about hearsay or what some agenda-driven writer claims.)
I have, and guess what? The writer of the article is flat out lying! The law contains no language whatsoever that outlaws homeschooling.
In fact, the law expressly allowed schooling other than by state schools:
§ 5. Erfüllung der Volksschulpflicht. (1) Zum Besuch der Volksschule sind alle Kinder verpflichtet, soweit nicht für ihre Erziehung und Unterweisung in anderer Weise ausreichend gesorgt ist.
Rough translation: All children shall attend primary school, unless their education is sufficiently provided for by other means.
Link to the full text of the law: http://www.verfassungen.de/de/de33-45/schulpflicht38.htm
The text is in German obviously, but you can run it through the Google translator or somesuch.
Note also that the compulsory schooling expressly concerns only primary school - grades 1 to 4. Secondary schooling was not compulsory under that law.
Lesson: Don't believe everything you read on the internet. Especially when written by people with an agenda, who will just lie if it suits their purposes.