I’ll be nitpicking but I’m not sure they are really that related.
I’m an introvert and my wife have SA. TBH it’s pretty different : if I go to an event where I know nobody or where I don’t feel like I have something to say, I’ll just go and keep politely quiet. I’ll get more bored and lost in my own thoughts or just listening without talking than stressed.
My wife on the other hand is like you said, terrified and exhausted by such settings and even with familiar meetings. Moreover, the SA doesn’t stops when you start interacting while introversion can disappear or become unnoticeable once you are integrated to the discussion.
Also I feel like introversion can somehow be controlled as you age while social anxiety don’t or can get worse.
I’m In my 30´s, I’m still an introvert but I learned how to interact with strangers politely without giving a fuck. My wife with SA clearly can’t do that at all and is always asking herself if what she just said was appropriate.