I've been using my 4090 with a Corsair ShiftX which comes with a 2 x 8 pin PSU connector, Not the same as 8 pin PCI-e, To 12VHPWR for nearly a year now with no issue, Work mate has been using his 4090 with the stock 4 x 8 pin connector since the 4090 released with no issue.
Unless a cable or connector comes directly from a company that has expert electrical engineers, Like a PSU manufacturer, I would never trust my £1500 GPU to a company that sells vastly overpriced cables that are of fairly average quality, Nevermind an adapter that has proven time and time again to be of questionable design.