Don't tell me that it's easier to RE an entire multi-megabyte messenger app than it is to real the source code. Assembly can lie to you as well. There are all sorts of ways to trick IDA and friends.
One does not preclude the other. For instance, the current Signal implementation is almost certainly prone to remote code execution.
How does the Signal project handle reports of potential vulnerabilities? I haven't seen any security contact information on the OpenWhisperSystems site.