Well, the thing is, that people learn new modules in different ways. Not everyone tries to consume a 400 page manual at the start of their journey. Some use training missions or YT vids to get started, then refer to the manual for specific advice, and gradually build up their knowledge base with further use of the manual, etc.
Now, AFAIR, there is no mention of the RAD ALT to be switched on in either the Start Up TM, or in the manual's Cold Start Procedure, although I might be wrong on that - not any YT Start Up videos I've watched, so it's understandable that this question has come up.