There are two flaws in your logic: 1. Security is not a binary. One system can be more secure than another even if it's not totally secure. 2. It's impossible

There are two flaws in your logic: 1. Security is not a binary. One system can be more secure than another even if it's not totally secure. 2. It's impossible in practice for any nontrivial system to be completely free of all vulnerabilities. When people talk about systems being secure, they generally mean free of known vulnerabilities.