Sounds like they have both swapped hot and neutral, and they are switching neutral. This is probably one mistake, made in the switch area.
This device is simple: the green light is between hot and neutral. The blue light is between hot and earth. The red light is between neutral and earth.
The device has a useful feature: it deliberately doesn't try to explain the meaning of every color combination. That's because there can be several reasons for any of them. And as such, anything they say is likely to be balderdash and more misleading than helpful. This drives us crazy with the similar tool in North America; some of us call it the "magic 8-ball" tester because their explanations of the cause are so laughably wrong, and waste a lot of time.
On safety, raises hands yeah, you got us. The system is overbuilt so it takes more than one simple failure to create a hazardous condition. All this stuff is redundant. Fuses, your system will work without em. Grounds -- well, you need them on that circuit, but not normally. Yeah, so shortcut away. What could possibly go wrong?