What is the soundest approach to al-Tirmidhi’s “we know it only through this route”?

Hasan li-Dhatihi, Sahih li-Ghayrihi, and al-Tirmidhi’s Composite Terms

• Treat it as a statement about limited known routes, then investigate the isnad and terminology
• Infer that all relevant routes are automatically weak
• Interpret “route” exclusively as a count of Companions
• Infer tawatur from one known isolated route