I guess that applies to people who were born deaf or lost their hearing at a very young age. The consequence of this is that they grow up with Sign language as mother tongue. This does not mean that they can not grasp complex concepts, but sign language is not the signed equivalent of the spoken language used around deaf people. Therefore, the written language they face is essentially the foreign language writing system, and complicated words and speech figures are more difficult to understand than in their mother tongue, that is to say the sign language.
The relevance of layouts and complex menus is a little less obvious. I guess these aspects are mentioned because anything that makes the reading process more difficult should be avoided. The reading process is already more difficult for Aboriginal signatories than for people who have learned the written language as a system that represents their spoken language.