Supermarket opening times are definitely not that restrictive (these days, but I don't recall it ever being like you mentioned & I moved to Berlin in 2013). The ones near me are usually open early morning till late evening (8-10pm), monday to saturday.
There is a lot of regional variation, mainly between the south and the rest of the country. 8am to 8pm monday to saturday is typical for Bavaria (Munich, Nuremberg).
The way you call 8pm "late evening" confirms GP's experience :)