You should reread the part I quoted above. It defines the terms used in §18/19: It’s §4(2) https://www.gesetze-im-internet.de/arbnerfg/__4.html it doesn’t talk about payroll or not payroll, it’s all about how related to your work assignment the invention is. The rules and regulations around calculating a fair compensation even grade on that exact metric among other things. So it’s essentially what the work contracts stipulate: inventions related to your work belong to your employer, even if done in your off time.

The problem in IT is that depending on what you do, everything might be related to a varying degree.