The specs list it as "1 x PCI Express 3.0 x8" Is that an error?

The specifications state that it has a connector labeled as '1 x PCI Express 4.0 x16 Female 90-Degree Left Angled (to Add-in PCIe Card)', not PCI Express 3.0 x8. So it seems the specs list you mentioned might be an error.

Questions & Answers: PCIE16L20-V4 200mm PCI-Express 4.0 x16 High Quality Flexible EMI Shielded Extender Riser Cable - Left Angled, Wide Range Vertical Mounting Gaming/GPU Support up to 4.0/16GB. OEM/ODM Welcome!

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Item#: 9SIARU1F7P4498
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PCIE16L20-V4 200mm PCI-Express 4.0 x16 High Quality Flexible EMI Shielded Extender Riser Cable - Left Angled, Wide Range Vertical Mounting Gaming/GPU Support up to 4.0/16GB. OEM/ODM Welcome!

High Quality Flexible EMI Shielded Extender Riser Cable;
PCI-E 4.0 x16 Compatible;
Support 4.0 / 16GB;
Rigid PCB and Standoffs;
Minimizing Disturbance & Degradation on Performance;
Wide Range Vertical Mounting GPU Support.

$60^.99 –