The term miqdār here refers to scholarly standing, mastery, critical skill, and recognition in hadith transmission. A highly regarded master may learn a particular report from someone of lesser scholarly rank without diminishing his own stature.
Example: A celebrated hadith critic acquires a specific well-attested report from a comparatively little-known but credible student. Even if they are similar in age, this can be senior-to-junior narration in scholarly standing.
Authority is report-specific: the person with reliable access to an item may serve as its source. But a renowned scholar's decision to narrate from a pupil is not automatic authentication of every report transmitted by that pupil.