There is no other source for estimates of their financial holdings than those attestations. For years, Tether has promised that they are working on an audit. No audit has emerged. This is suspicious because audits aren't particularly hard for legitimate organizations.
People have gone looking for evidence that Tether really has as much as it has said. Nobody has succeeded. For example https://www.bloomberg.com/news/features/2021-10-07/crypto-my.... Which, after noting Tether's settlement for past fraud, and prominent executives with suspicious criminal histories, famously said that Tether "is quilted out of red flags."
And yet, many strongly believe that Tether is for real. Given that nobody has produced evidence of their wealth, I regard this belief as a question of psychology.
Said psychological belief has a simple explanation. We humans have a strong tendency towards cognitive dissonance. Meaning that we reject evidence which questions our sense of identity.
If I'm a bitcoin millionaire, my likely identity includes, "I'm a millionaire." But the financial system does not allow us to directly trade bitcoin. What I can actually trade bitcoin for is Tether. And therefore my belief in my own wealth relies on trusting that Tether is equivalent to money.
How can I support that trust? I can point to someone smart and say, "They believe in Tether, so it must be real!" I can point to someone rich and say, "They believe in Tether, so it must be real!" I can get angry at anyone who doubts Tether. It is easy to observe all three behaviors from people into Crypto.
That Bloomberg article offers an ironic example. Back in 2021, many crypto enthusiasts pointed at Sam Bankman-Fried. Who was both rich and smart, and publicly believed in Tether. Therefore he was quoted on the topic. Today, of course, he is in federal prison for having run a Ponzi scheme. If we behaved logically, any crypto enthusiast who used to look up to him, should be saying, "Wait, am I sure I'm placing my trust in the right place?" But, predictably, few do that. Instead they dismiss this past mistake, and place their trust today in someone else.
Now everyone knows that crypto is full of Ponzi schemes on top of Ponzi schemes. People are often OK with trying to take advantage of the greater fool, because they don't believe that they will be the greater fool. Others see people apparently making money and get in, driven by FOMO. But statistically, most people in a Ponzi scheme will lose money. And yet they will look like really smart people - right until the scheme collapses.
So here is the real question. Does Tether really have the money? Or does the entire infrastructure of crypto Ponzi schemes rest on yet another Ponzi scheme?