It's absolutely true that this probably won't power a 4090/5090, and you're likely pushing your luck with a 5080.
Not entiely true. Yes for the 5090, but the 4090 and 5080 is not a problem...unless you use a >250W CPU or use 12+Fans with a lot of PCIe 5.0 M.2s at the same time.
I use the old ATX 2.41 version of the SF750. I used it with a 450W 4090 and a 105W 5800X3D just fine. (even a 5950X with 142W before)
It was in a "small-ish" NR200P.
I also OCed my 4090 with 600W for testing and the PSU worked, but the PSU fan ramped up and was very loud.
To use it with a 4090, I used the Nvidia adapter and later I bought the adapter from Corsair, the unsleeved Type4 2x8-Pin to 12VHPWR. I run my GPUs with that cable for 2 years now without issues.
My current PC changed a bit. Now I use a big ATX case, because NO AM5 ITX or µATX Board has the features I wanted.
It still uses the same PSU, but my CPU is now a 9800X3D and a 5080 (sold 4090 last year). PSU still runs fine and pretty much silent.
I also use 3x M.2 and 3x Sata SSDs +6 Fans but no RGB.
P.S. I live in Europe and use ~230V power...