It looks like UCSI should be exposed via ACPI device PNP0CA0, and should report a specific GUID via _DSM. Looking at the hx20's ACPI tables, that is certainly what’s happening. Everything checks out on the ACPI side… including the EC communication through ER1/the “customer memory map”.
EDIT: with dyndbg tracing module acpi +p:
[ 4103.506382] ucsi_acpi USBC000:00: Adding to IOMMU failed: -19 (ENODEV)