Ive been told that it is good to follow "80% rule" when using ATX power supplies, but for server supplies I can nearly max out no problem.

Ive been wanting to use a Corsair 1200W power supply (platinum) for my machine learning server, because it is already installed, but I added another 3090. It will most likely be doing a lot of deep learning months at a time. It will be running at 1130W, about 1300W from the wall.

Is this a mistake?

Nowhere in the manual has the corsair psu said I shouldnt be running at close to maxload for a very long time. It also has a 10 year warranty, which makes it confusing even more.