People who are mute still communicate. They're not merely listeners.
If you're not practicing output, you're not truly comprehending. Output, whether it be speech, writing, or signing, is how you get and demonstrate confirmation that you comprehend.
If you were speaking your native language to someone who understands it, but spoke back to you in their native language that you are learning, you can get the same confirmation. I've done this before and it can work quite well. So I'm not really convinced it's necessary for comprehension. It's necessary for being able to speak of course, I don't disagree with that.
This also implies you cannot understand a language without people who can speak it to you. You can learn to read books without reading them out to someone who tells you what they mean.
Honestly, this doesn't even really make sense considering my own experience of going from not comprehending stuff like shows and podcasts to being able to almost completely understand everything being said a lot of times, without any real practice speaking to people. Unless you want to claim that I'm not actually understanding anything at all :).
If you're not practicing output, you're not truly comprehending. Output, whether it be speech, writing, or signing, is how you get and demonstrate confirmation that you comprehend.