This pattern expresses a reluctant or cautious affirmation: 'It's not that one cannot say...' meaning 'It can also be said that...' but with a nuance of 'there is some merit to that view, though not entirely.' It's a double negative: 言えない (cannot say) + もない (not even) creates a softened assertion. You use it when you half-agree with an opinion but want to hedge, often in discussion or when conceding a point. It's a written, formal pattern, common in essays, critiques, or polite debates, not in casual chat. The quoted part is a full clause: a statement you think could be made. For example, if someone says a movie was boring, you might say 面白いと言えなくもない (It's not that you can't say it was interesting) meaning you see some merit but not fully. The tone is intellectual and reserved.