My theory is, that they are mostly not backed by real USD nor assets - it's virtually impossible that they have so much commercial paper without nobody in the business even knowing them. They are most probably lying.

If this is true, and everything points to it, then there is simply much less real USD on the exchanges than people think. People often quote the crypto market cap and that Tether only represents 5-10% of it - but most BTC were never really available anyway - they were never bought or traded on any exchange. So the total inflow of USD (or any other real money) is much less than crypto market cap - and a lot of it was eaten by miners, arbitrageurs and exchange fees.

When the music stops - and enough people try to get their real money from the exchanges, it simply won't be there.