The teacher who has stopped learning has stopped teaching — they are merely distributing the sediment of understanding that has ceased to be alive. The deepest learning almost always happens in the attempt to transmit: questions from students reveal the gaps in one's own understanding; the demand to explain clearly forces precision that mere knowing does not require. The Sutras' transmission is itself evidence of this: Patanjali did not write a treatise for himself but for the student whose questioning sharpens the teacher's own understanding of what, exactly, citta is and how it moves.
Each step builds on the last.