No one knows what Giuliani said in this interview, so any assumptions about him turning on Trump are baseless. Obviously, Giuliani had knowledge of the plans to present alternate electors if VP Pence didn’t certify the questionable election results. He also could’ve explained that Trump was concerned with evidence of voter fraud that was being shown to him.
Few people were as close to Trump during his attempts to overturn the election or have done more to spread Trump’s lies about the 2020 election being stolen than Giuliani, so it’s no surprise he’s among a host of people being questioned by Smith. If he chooses to be on the right side of history and come clean about Trump's nefarious plans, he could save both himself and American democracy.