The burden of proof lies with whoever makes such a claim.
Listen, a DDC is essentially a USB isolator (many ddc has it now for electrical noise) combined with a reclocker (for jitter reduction), digital format conversion, etc. It does not matter how many stages of isolation or reclocking you add; quality matters far more than quantity.
From an engineering standpoint, there is no obvious reason to expect benefits to accumulate indefinitely. Once jitter is reduced below the DAC's sensitivity and galvanic isolation has already broken the electrical connection, additional stages often yield diminishing returns. That does not prove that no system can ever benefit from an additional stage, but it does mean that claims of large improvements should be supported by measurements over your subjective listening tests.
The lack of benefit from cascading DDCs was demonstrated back in 2021. You chose not to believe it, and that is your prerogative. Continue doing what you are doing—no one can influence those who believe they are smarter than everyone else when, in reality, ideology has gotten the better of them.
However, while you have every right to express your views, we also have the right to voice our concerns when we believe misinformation is being spread. If your goal is to persuade others, you would be better served by providing concrete evidence to support your claims rather than relying on instinct, assumptions, or personal belief.