All topics added to a 1on1 agenda are visible to both people by default. You can make a note to self on a 1on1 agenda that is not visible to the person with whom you are having the meeting.
First, look at the avatars at the top of the page -- if the other person is on the agenda at the same time, you'll see a green circle and dot on their name. If they're in the meeting now, wait until they've checked out to add the topic. After adding the topic, hover over the row, click the three dots icon (kebab menu) at the end, and select Make private. This topic will now only be visible to you. (Super secret item and want to be super sure? Click the three dot icon (kebab menu) again to see if Share topic is available; if it is, the topic is private.)