i dont understand why it's referred to as "communication disorder" when it's pretty much just social anxiety
Well, a quick search gets us
A communication disorder is any disorder that affects an individual's ability to comprehend, detect, or apply language and speech to engage in discourse effectively with others.
Since social anxiety is also considered a disorder, if that's what we're going to assume she has going on, hers would be so bad that it's certainly having a negative impact on her ability to speak with others. With that in mind, we can probably call it a communication disorder based on the basic definition right there.
Now, it might not usually be considered as one outside of this interpretation of a fairly specific context, but we should also bear in mind that this series is not something with a serious technical sort of deal going on in it, and is largely narrated by teenagers who are anything but professionals in the fields that would know the specific ins and outs of the real deal. Nor is it particularly uncommon for mangaka to use somewhat superficial knowledge of things in their work. If I could build an argument for calling it a communication disorder in less than two minutes with Google-sensei, so could they.