It absolutely could be that the democrats on the court were voting based on partisanship. It's just not so clear because their dissenting opinion isn't so hypocritical.
The state legislature's power to choose electors for the presidential election is plenary. If they wanted to they could simply appoint electors without even holding a statewide election of any kind. They could draw names from a hat if they wanted. It's none of the Federal Supreme Court's business how a state chooses it's electors. Once the legislature writes laws governing the selection of their electors it is then up to the Florida courts, and ultimately the Florida Supreme Court to interpret those laws, except where they violate the U.S. Constitution or federal laws that stem from powers granted by the U.S. constitution.
The majority's opinion was held per curiam which means it is held in the name of the court, rather than having individual justices sign it. Per curiam opnionsare normally short, deal with non-controversial issues and are unanimous. The majority opinion also contains this zinger:
Our consideration is limited to the present circumstances, for the problem of equal protection in election processes generally presents many complexities.
Even the majority realized that this went against their principles and would come back to bite them in the ass. No problem, though. They got around that by taking the unusual step of saying, "Oh....this is how we're going to decide this case, but we're stating for the record that we are not setting a precedent here."
How convenient. They get to have their cake and eat it too.