First, I disagree that Russia saw Ukraine as turning into a long war. They thought they were going to blitzing into Kyiv and force a change of government. They thought they would do the 1968 Prague thing and it would just be over. He did not expect to still be fighting almost two years later.
Second, if they can’t manage to properly invade a country on their own border after being able to set their own timelines, position their troops and plan supply lines, and draw up plan after plan for contingencies- there’s no way they open up a second front, especially against a NATO country.
This is just saber-rattling in order to try to scare European countries from continuing to send weapons to Ukraine.