Why should a known mudallis not be included without qualification in Muslim’s presumption?

Muʿanʿan Transmission: Possible Meeting, Evidence of Hearing, and Continuity

• Every narration of a mudallis is fabricated
• Mudallis always means unreliable in every respect
• A mudallis cannot be contemporary with a teacher
• Tadlīs raises a specific risk of an omitted intermediary behind ʿan