I respectfully disagree with your assessment. You are referencing the quote "It's not appropriate to use the aside element just for parentheticals, since those are part of the main flow of the document." However the OP specifically said that they are looking for a semantic element for "a note that may be useful to read at a given point of a tutorial, but is not part of the main tutorial flow". That is what "aside" is for. It's not part of the main content flow.

Of course how each developer interprets and applies these very generalized guidelines is subjective and will vary from person to person.