>> So if Tether had say, $50-billion in 10-to-30Y treasuries at the start of the year, they only have $45-billion of that now.

This is nuts - they dont and it wouldnt make any sense. You cant have a short term cash-equivalent backed with long-duration bonds. It would be a total asset-liability mismatch.

For reference:

T-bonds mature in 20 or 30 years and offer the highest interest payments bi-annually.

T-notes mature anywhere between two and 10 years, with bi-annual interest payments, but lower yields.

T-bills have the shortest maturity terms—from four weeks to one year.