For good or for bad, Trump's opinion on anything is the opinion of whoever he just talked to. I too am cautiously optimistic when he says things like this. But I think some people in his cabinet are firmly pro-Kremlin, and if they talk to him and convince him that Zelensky is the bad guy then he'll switch again. He's made fake threats to Putin numerous times now, like the infamous "Vladmir, STOP!" tweet. But they never manifest to anything. Both Putin and Netanyahu know they can jerk Trump around with meaningless words for another few months while they continue any aggressions they want.
https://i.imgur.com/dzGMd.png