Is the questioner considered sinful for deluding a non-Muslim into believing it is possible for him to enter Paradise despite not being a Muslim, relying in this on his previous ignorance of the religious ruling, especially since he cannot correct the information for him now?

Your statement to the man: "I do not guarantee myself in Hell or in Paradise, nor do I own Hell or Paradise to say where I will be or where you will be," carries a valid meaning, which is that Allah might guide him to Islam and he enters Paradise. However, if you meant that Allah would admit him to Paradise even if he died a Christian, then this statement is false. As long as you were ignorant at the time and had a good intention, we hope there is no blame upon you. If you meet him again, explain to him, with wisdom and kindness, that Islam has abrogated other religions and that it is the accepted religion.