Commercial paper has very short maturities, so unless there is a wave of bankruptcies it is hard to take losses. You don’t sell CP, you just stop rolling.
Chinese commercial paper had a large wave of delinquencies earlier this year (https://www.reuters.com/markets/rates-bonds/china-commercial...). The chances of Tether's holdings having zero defaults amongst all of that is... low.
Tether has an easy fix here: release the "frequent professional audits" they claimed on their website as far back as 2015 (https://archive.ph/mVPmL). They've provided various excuses and called attestations audits for nearly a decade now; their credibility is shot.
Read the tweet again. What they wrote is perfectly true if they rolled off 30B and are sitting on 10B of trash that needs a huge loss provision. They never say they liquidated all their CP holdings. I suspect that tweet is truthful but completely misleading.
They do separately say that; https://tether.to/en/tether-slashes-commercial-paper-to-zero....
(They have, of course, repeatedly lied elsewhere on that website.)