Just to be clear: you're asserting that "there are some inalienable rights" can be debunked by the existance of one that is not inalienable?
That's not how this works.
We're talking about whether people should be permitted to sign away their right to speech. I think you've conceded that such is permissible at least for a limited duration. Shall we quibble the permissible durations, or are you done?
You asserted that free speech was an inalienable right, they provided an example showing it's not.
They also could have mentioned: NDA's, hate speech, threats, incitement, purjury, defamation, security-clearances or state secrets.