What isn't important here is that any of what Putin says is plausible, but, rather, that he says it. In the words of Aleksandr Solzhenitsyn:

“We know that they are lying, they know that they are lying, they even know that we know they are lying, we also know that they know we know they are lying too, they of course know that we certainly know they know we know they are lying too as well, but they are still lying. In our country, the lie has become not just moral category, but the pillar industry of this country.” //

A Ukrainian attack on a soft target like a concert, while not on the scale of the Russian attack on a theater in Mariupol, Ukraine, on March 16, 2022, that killed up to 600 civilians, could possibly create the national outrage needed to support increased mobilization.

Given the laser-like focus of the Russian government on blaming Ukraine, you can't help but recall the apartment block bombings in 1999 in Russia that killed over 300 and injured nearly 1,000. Those attacks were immediately blamed on Chechen guerillas, but it became clear that the FSB had carried out the bombings to create casus belli for the Second Chechen War.

To the extent this propaganda campaign is intended for the West, it will be used to justify increased attacks on civilian targets in Ukraine.