YABU, that's the biggest load of bullshit I've seen in quite a while (even some of the stuff I've written read on Mumsnet).
Social anxiety is a medical condition, whether you agree with that or not (and I suspect, perhaps unfairly, that you are not trained as a doctor). Recognising it as a medical condition allows the sufferer to get help, to get their condition treated. Perhaps just as importantly it allows the sufferer to realise they are not "mad" or "fucked up" or anything else. They are ill and their illness can be treated.
This "pull yourself together" attitude does not help someone overcome their anxiety in the slightest. If tells you they cannot cope with life or that they are feeling suicidal, telling them to "pull their finger out" will not help them overcome their feelings - very much the reverse.
Anxiety needs to be worked at to be overcome. It is very hard work too. One needs to understand the reasons why one feels anxious, what the triggers are. Then to explore the actual thoughts and feelings that it generates. Then to gradually expose themselves to the very situations that they are anxious about - no matter how big or small these situations may be. This is an ongoing job, a daily grind is needed in order to minimise the risk of slipping back into old habits.
Ignoring mental health issues does not usually mean that they will go away. Problems need to be recognised before they can be addressed.
Denying that anxiety is an illness is like telling someone who has just had their legs amputated to "pull themselves together" and just get on with learning to walk with prosthetic limbs. They need assistance to do this, they gradually build up the muscles that are needed and slowly learn to walk again. It would be pretty unhelpful if the doctor chucked a couple of artificial legs at the patient and told them to just get on with it.
Whether an illness is mental or physical, the basic principle is the same: recognition of illness is needed to allow the correct treatment to overcome the illness.