Language comprehension is a two way street. If someone thinks comprehension is a passive activity, then they're likely not fully comprehending. Comprehension requires active engagement.
When you're learning another language, there'll be countless times where you're like "ohhhh, I understand this." Then the moment you open your mouth, you'll get pretty directly told, "No, you completely misunderstood." You'll also encounter people who immediately correct your mistakes when speaking, whether they're grammatical or misusages of words.
As an example that people here might better grasp, learning a language without speaking is like learning a programming language without writing your own code. You might generally recognize the syntax and usage of various functions, but you won't truly grasp it and understand its nuances until you sit down, write, and accept that you'll make mistakes and identify them.