So the video where he said that, was published on April 2017 and he said that when the interviewer talked about end-to-end encryption, chat clients and free software. Presumably he was referring to “problems related to security/privacy” that Signal app has or had.
I searched several times on the internet for the “problems” Signal might have and could not find any. Sure you may not know what he meant, unless you can ask him directly, but maybe someone knows of any issues of Signal concerning privacy or security?
Any answers appreciated, thank you.