For clarity: this is about different varieties of Standard German. It does not apply to other Germanic varieties* also spoken in the DACh (like Alemannic, Austro-Bavarian, etc.); those evolved alongside Standard, not from it.
I don't know enough about the situation of Standard to know if Dollinger's claim is true or false. However, it feels to me like it's a bit of bullshit, based on anecdote - I had German classes in uni, and one of the professors was a native Austrian speaker. As teaching she often highlighted differences between how Austrians vs. Germans would say something, and that both forms were correct - but you shouldn't mix them. For me that signals divergent standards, so a pluricentric view. But a sample size of one is not that useful, so... take that with a grain of salt.
I'll tag @[email protected] here, as he's likely way more informed about the topic than I am.
*note: "variety" = "language or dialect".