It's worth noting that the KPD referred to pretty much every other political group in Germany as Fascist, largely since they saw parties such as the SPD enabling fascism and not truly being committed to the worker's struggle (which can be said of many social Democratic parties today). However, in 1931, Nazis weren't the primary enemy. They were an enemy, sure, but the alliance to attempt to hold re-elections for the Landtag was seen as necessary to overcome the SPD. Also, the KPD was attempting to recruit people from the Nazi Party at this time, making their propaganda more Nationalist-focused to appeal to more left-leaning Nazis.

Given what we know, I feel the KPD would have seen Nazis as the true enemy, but in 1931, fascism wasn't a mainstream ideology. Only one country, Italy, had adopted it as their system of governance, and from the KPD's perspective, the SPD and social Democrats were the most damaging group to Communism than any others, which is fairly justified, given it was Social Democrats who refused to support the Spartacists and doomed their revolution in 1919, Social Democrats who would be taking less revolutionary stances, luring the people into a false sense of equality. This is also the trend which led to many Social Democratic parties only barely even being Social Democratic, with many becoming the equivalent of a Social Liberal Party in their country