Temporary tablespaces are full of junk, so RMAN doesn't back them up. When you do a recovery, the system metadata about the temp tablespace is present, but the file is not actually recovered, so you need to create one. That is what this step is doing.
You should never follow anything blindly.
You need to convert your single instance database into a RAC database. Those are the steps you need to follow. The poster is creating a 4 node RAC, so many of the commands are run 4 times (1 per node). Your situation will be different for obvious reasons.