You make it sound like the center party is playing both sides but as I understand it is not part of that block, and they would be OK with social democrats but not far left in government. What's wrong with that?
However, the left has stated that they rather reasonably won't support a government that they are not part of. The center party on the other hand has said they won't support any government that the left party is in and seem to be open to forming a government (at least not denying when being asked directly) with the right wing parties of the former government, despite that bloc losing the election.
When one party's "red lines" only seem to soften when facing the right, I think that does say something about their true intentions.