> The fact that the B650 can even do 5.0 on the GPU lanes (it is able to do 4x 5.0 on NVME) makes me wonder if the B650 chipset is the same silicon as B650E and the speed is just really a configuration setting.
All the AM5 chipsets are the same silicon, except A620 which has less capability (might just be fused off though) and the B840 which seems to be a rebranded AM4 chipset. B650, B650E, B850, and X870 are all one Promontory 21 chipset; X670, X670E and X870E are two Promonotory 21 chipsets (one chained behind the other). The B840 seems to be the Promontory 19 silicon, same as a B550 AM4 chipset; which seems weird, but it has more downstream connectivity than the A620 chipset, although the cpu to chipset link runs at pci-e 4.0 for A620 and 3.0 for B550 and presumably B840, so that's a bottleneck if you have high throughput devices behind the chipset.
The GPU lanes on AM5 all come direct from the CPU, as well as at least one x4 m.2 socket, and there's 4 more CPU lanes not earmarked by AMD. Those don't touch the chipset, and any correlation between the chipset branding and the speed of those lanes is a marketing restriction: If you put B650E branding on your board, AMD says the board must support pcie 5.0 for the GPU and first NVMe slot; but the chipset isn't involved in those lanes, it's just a marketing agreement; I'm not sure if AMD disallows using pcie 5.0 on a B650 board for the GPU slot, but capability is a matter of trace design between the slots and the cpu and maybe what the board maker is willing to certify / what the firmware is willing to enable. For B850, AMD says pci-e 4.0 for the GPU and 5.0 for the first nvme.
I'm sure there are some X870 boards that are significantly different from B850 boards, but there's probably a lot of boards where everything is the same other than the product name. Likely the same was true of B650 and B650E. For reliable PCI-e 5.0 operation, you do need to follow proper design rules for the traces, and that results in a more expensive board than following the design rules for PCI-e 4.0 operation. But it can make product sense to sell some of those 5.0 capable boards with a lesser chipset brand anyway.
The physical motherboard PCB needs to support PCIe 5.0 speeds and be tested for it at the factory.
If their hardware team designed the PCB and the qualification process to the specifications of the B650 chipset, they would have targeted PCIe 4.0 speeds.
There's no need to test for it at the factory if you're not promising it works. Certainly, the PCB needs to be built for it; but I'm sure some boards are designed for B650E and then put together with a B650.
Regardless, the chipset has been out for almost 3 years. Kind of late to put the horse back in the barn. Regardless of certification, lots of systems are out there where it's enabled, and those with those systems and a pci-e 5.0 can tell us how it works regardless of what the branding says. Maybe it's flaky, maybe it rarely works. Maybe it just made sense to follow the pci-e 5.0 design rules for the gpu slots, even without the branding; maybe the pci-e 4.0 design rules are good enough at the trace lengths on a typical am5 board. Maybe once you have the PCB designed and built to run the cpu m.2 at 5.0, it doesn't cost anything to follow design rules for the cpu x16 slot as well.