I agree with this statement in some respects, and do not in others.
I do agree that toxic shame could be the original cause of social anxiety, in some cases. As a child, I was bullied severely by my fellow peers, both inside school and outside, through three different schools. In result, I could not help but to come to the conclusion that I must be defective, that I was a mistake that should have never been made, which caused me to isolate myself. Afterwards, I, basically, put away the real personality and memories of me, and created a new person, who appeared untouchable and perfect.
However, I do not agree that social anxiety is merely a symptom of toxic shame. Toxic shame caused me to attempt to become a new person, one that appearred perfect and untouchable, but did not prevent me from being in social situations. Of course, I would have that old person inside, the original me, who would be distressed and anxious, who would want to be alone, but I had invented a new person, which meant that old one did not matter anymore.
Of course, when people decided to, also, not accept the new me, that was when the facade began to crack, and the original me began to surface, once more. And that was when the social anxiety truly arose, part caused by feeling defective, even more so now because, even though I had evented a new version of myself, even that version could not be accepted, and part because of the shame, the shame that people would look at me, and laugh and talk and point, knowing I was defective.
And still, the problem remains, today. And I do not believe that it is the toxic shame that must be cured, but rather the social anxiety. Toxic shame may be a cause for social anxiety, and can be part of social anxiety, but I do not believe that it will ever become a problem worse than social anxiety, social anxiety becoming merely a symptom. Toxic shame results in you avoiding people in general out of choice, social anxiety results in avoiding people as mandatory.
But, each to their own.