They're talking about the Android app. In Android the application which sends messages can be replaced, and so Signal offers to replace it, as a result it needs to be able to send old-fashioned SMS messages when the correspondent doesn't seem to have Signal.

Your iPhone doesn't let anybody do that, just like it doesn't let Mozilla provide a different web browser and so on. This has upsides and downsides for security which I'm sure you've already thought about, for me it's a good reason not to own an iPhone, but the rationale for the opposite decision is sound.