I don't think anything says that it isn't plausible. The issue is that $450M appearing with no third party audit in a relatively short time period with no fanfare (e.g., Tether: "We just secured a new deal with a bank and have issued more tethers as a result!") is a bit odd to look at from a normal perspective. If Tether were 10 years old and had established chains of successful audits, this would be less of a big deal (but still a deal).