Let's get real, shall we?
Putin's KGB training was to exploit useful idiots in other countries. He's done this with any and every US interviewer, because NONE of those interviews tells anything surprising. What, he's going say, "well, ya got me!" and confess to all the murders, unjust jailings and imperialistic BS under his quasi authoritarian rule? No. Never has, never will.
Carlson is still butt hurt over getting the heave-ho from Fox. He craves that public recognition, the need to be touted as a voice of knowledge and authority. The fact that his "voice" drips with the venom of racism, bigotry, fascism and misogyny is not a problem for him.
Nor is it for Putin, because he knows that giving the spotlight to Carlson will further sow the seed of discourse in America....which can translate to congressional/White House actions.
Now, if we're dumb enough to put Trump back in power, granting Carlson's little interview will not have been in vain.