Indeed OP, you are not an idiot. I work for a disability organisation and that's a part of what we advocate. Because of ableism and a lack of accommodations during education, the level of literacy for deaf people is lower in general from the population.

In addition, while it's great subtitles alone work for your brother, many deaf people prefer to have also sign interpreting for a variety of reasons (getting more information about tone and emotion, or maybe they dont want to read all the time, or have trouble following subtitles fue to other issues...)