It's okay that you aren't familiar with PEGs. I am familiar with PEGs, and so I know just by reading that, which way the ambiguity resolves. So for me, a human reader, there is no obvious ambiguity.

It's like saying there's an ambiguity in "not a and b". Sure, if you don't know the precedence assigned to 'not' and 'and' in your language. But you're supposed to learn those things.

Your not knowing how PEGs work is a weak argument against using them.