The sentence explaining what it does has a typo which makes it confusing: > Unclack is the small but mighty Mac utility that mutes your keyboard while you type. This should say "mutes your _microphone_ while you type". You can't mute a keyboard
keep in mind mute does not only mean "to silence" (verb form). It's also "deaden, muffle, or soften the sound of."
So, practically speaking, there are very few physical objects that produce sound that can't be muted.