Dean (the author of the article) mentioned that several other apps have attempted to use peertalk and other OSS code this app uses but had their app rejected by

Dean (the author of the article) mentioned that several other apps have attempted to use peertalk and other OSS code this app uses but had their app rejected by Apple (e.g. see mronge's comment). So perhaps the lack of attribution was made with the assumption that use of these OSS packages was part of the reason for rejection. If this is not the excuse then I can't imagine any reason for it. Attribution in this form is a simple footnote somewhere and giving credit to others never hurts your own brand. In fact I'd argue it contributes to it.