what should you answer to your potential supervisor's question: Are you applying to other programs?
The obvious answer to the first is that you will apply until you are accepted into a suitable and interesting position. For the second, it has no implication other than that he is interested. He is willing to give you advice, but your admission can depend on several things. The SoP is only one of those.
In some cases, depending of place and field, the professor may not be directly involved in decisions to accept you. But nothing is assured until you get an acceptance letter.