That's partially true...
It does natively support 3.0 but is not 3.0 certified.
If you check the data sheets it satisfies 3.0 but Intel did not get it officially certified for 3.0, it's the lack of cert is why nVidia has you force it. But SB-E has the full 3.0 bandwidth on supported motherboards.
It was a weird time, transitioning from 2.0 to 3.0, not all motherboards BIOSs supported it, not really any 3.0 cards.
All comes down to the certification, can't officially call it 3.0 without it whether it's fully supported or not.
page 10... 8GT/s (3.0 speed)
Used to be annoying because they all have to support it. the Xeon counterparts were officially 3.0 certified but still needed mobo support to recognize the 3.0. One HUGE reason to make sure you go with mobos that get frequent BIOS updates/support
x79 is kind of an oddball platform, it's had it's growing pains, kind of similar to nvme support. Technically it's not suppose to work on x79 but is fully recognized in my BIOS now and works beautifully.