No, they also have third party attestations that it is fully backed.

Apparently those attestations are relatively recent in their history, but also not very convincing:

https://www.coindesk.com/markets/2021/03/30/tether-takes-ste...

"as noted later in the article the composition of Tether’s assets is not spelled out in the attestation"

Meaning they may be backed by loans or fraud coins or anything.