The above is the main reason I won't contribute.In my amateur opinion, I agree with Brigadier, and I thank him for explaining it so eloquently.
Rain water has the potential to inundate the tank.
I can understand that there are times when rain water is directed to the tank, but if it's avoidable, it should be avoided.
If a sceptic tank is possible, a soakaway. or other mechanism, must be equally poossible for the rainwater.
The above and the post it refers to are word salads. I know they are word salads, but the authors believe it to be true. If I was to contribute, my words would be poo poo'd (pun intended) so I don't bother, I don't want to be farting against thunder.
BTW, there is much more to water qualifications than septic tanks.
They start with the hydrological cycle, water extraction, water storage, filtration, what we put into the water, what we take out, why we take it out, Chlorination, Ph correction, sampling, transmission, distribution, materials and components, wastewater treatment etc.
That is just a snapshot.