As far as I can tell they make no mention of where exactly they are getting their data, but they claim to update multiple times daily and review all data manually.
They also say the following:
> Uns blieb bei der Aktienanalyse oft keine andere Wahl, als unzählige Geschäftsberichte der vergangenen 20 Jahre zu öffnen und dort die gesuchten Daten manuell und mühsam viele Stunden herauszusuchen.
The essence of which is that in the past they often manually extracted data from the last twenty years of annual reports from a given company. Maybe that's still how they're doing it.
They also say the following:
> Uns blieb bei der Aktienanalyse oft keine andere Wahl, als unzählige Geschäftsberichte der vergangenen 20 Jahre zu öffnen und dort die gesuchten Daten manuell und mühsam viele Stunden herauszusuchen.
The essence of which is that in the past they often manually extracted data from the last twenty years of annual reports from a given company. Maybe that's still how they're doing it.