Some terminologies in Wirth's book and the satisfying of deterministic algorithm
The definition of start symbol that you quote has nothing to do with the definition of start symbol in the context of deterministic grammars. Here is a correct definition:


  A terminal $a$ is a start symbol of a sequential form $\alpha$ if $\alpha \Rightarrow^* aw$ for some $w \in \Sigma^*$.


In words, $a$ is a start symbol of $\alpha$ if $\alpha$ generates some word starting in $a$.

In your example, $Aa$ generates the word $ba$, and so it shares a start symbol with $b$.

Why is sharing a start symbol bad? Suppose you are trying to parse $A$, and are encountering $b$. Which production do you choose? $Aa$ or $b$? It is impossible to tell without looking ahead. The deterministic algorithm considered by Wirth has no lookahead, and so this situation is problematic for it.