Only the CPU direct lanes are PCIe 5.0 and, being direct, none of that gets multiplexed. You either use all of a direct slot's assigned lanes at 5.0 speed or you miss out on that bandwidth.

Everything else not CPU direct (wired and wireless networking, SATA storage, non-primary USB ports, and 3 PCIe 4.0 addon cards) is connected via the chipset(s) which connect back via a single shared PCIe 4.0 x4 connection to the CPU. These are multiplexed but nothing here is using PCIe 5.0 bandwidth and even just 1 busy slot alone off this collection is able to consume the entire uplink bandwidth.