You're misunderstanding the rhetoric there - they're unambiguous by construction because they deterministically resolve earliest-match-first - a well-defined rule.
It's impossible to write an ambiguous PEG. Any grammar you write in a PEG is never ambiguous. A grammar you write using a CFG can be ambiguous.
As your own link says "Unlike CFGs, PEGs cannot be ambiguous".
Or read Ford's original paper https://bford.info/pub/lang/peg.pdf - they solve "the ambiguity problem by not introducing ambiguity in the first place".
(I wrote a much-cited thesis on PEGs and issues such as ambiguity in precedence parsing - I'm not just talking off the top of my head here.)
"A PEG parser generator will resolve unintended ambiguities earliest-match-first, which may be arbitrary and lead to surprising parses"