Germany is a lot of things. It's also changed radically in the last 80 years.
You've learned something in a book and you think it applies 100 years after the events you were reading about.
Not only that, instead of looking at current German politics, you've decided to look at old German politics from a simplistic view, and you've tried to make that fit your weird ideas about what is left and right.
You say Germany has "ultra-left communists" v. Christian Democrats.
No, you're wrong.
Germany has three left wing parties, die Linke, who are further left, maybe they even have Communists, they have left-Green who aren't Communists for the most part and you have the SPD which is a traditional left wing party, not Communist.
They do have a Marxist-Leninist Party. 18,000 - 22,500 votes this year and the German Communist Party, 5,000-15,000 votes this year.
So, Communists have their choices, they can vote viable parties, or they can vote Communist Parties.
Hell, you can even vote for the B* or BergPartei. They hold a vegetable battle in Berlin, they got 222 votes this year.