Yup. That was the day he said it. It seems too obvious
Look at all of Jesus sayings. He constantly said to people "Truly I say to you..." this and "Truly I say to you...." that. He at no time EVER said in any other part of the new testament "truly I say to you today" did he?
Only in when speaking to the so-called criminal on the stake did he use this expression and add the "today" bit.
That to me shows he was saying to the criminal he would be in Paradise today.