"Authority is contextual and constructed' is another pillar of the ACRL Information literacy framework. Simply stated, what 'counts' as scholarly depends on the context, and on a set of social constructions. These consideratons are fluid, they are not fixed. Some of the relevant context is about the mode of communication (twitter vs an academic article) and some of it is based on you (how you use the information, what additional informaiton you provide, what you are trying to say or do in your writing). Disciplines also have conventions and standards for what is considered scholarly or popular.
Less scholarly sources can be used to humanize a topic, add a storytelling element, bring it up to current day, as examples, to individualize a generalization, and for many more reasons. As long as YOU use them intentionally, with purpose, they are usable.