Well the OTL split is the key IMO. In TTL they reabsorbed them, which in due time meant that the "former" Communists together with the extremist wing of the SPD became the tail that was wagging the dog. It still calls itself the SPD, but is now the KPD in all but name except that people like the guys wife haven't realised it yet or are living in denial.Up to and including the OTL split of SPD / USPD.
So in short, Imo it portraits the SPD as more left then I had anticipated and that clashes with the characters shown.
That's something that happened plenty of time in history. A mass movement is unwilling to kick out it's extremists due to a combination of being seduced by the additional power and naivety about how they'll be one big happy family and decade or two later the extremists are in the drivers seat and the old guard is getting purged.