Meaning is not carried by the words, it’s carried by your mind. We assume that we assign the same meanings, but that’s a big assumption.
These online conversations, composed of words and only words, are arguably 99% us just talking to ourselves. Maybe even 100%.
No, we are communicating. People can coordinate their actions to achieve things that are impossible for an individual. We obviously don’t have perfect shared understanding, and miscommunications are not uncommon (as others have already pointed out) but we can exchange enough information to do useful things.
Also, we can make jokes. The fact that it’s possible to craft a joke and make someone laugh by setting up and intentionally subverting expectations through language is pretty good evidence that we have shared understanding and similar processing.