I could confirm is was a bios function limit or bug from DL20.
I get other ML110 Gen9 server ,and update it to least version bios, without insert any pci-e NIC card, it still show SR-IOV SW option in BIOS ,same as the documents say. but in DL20 Gen9 doesn't show it in same place.
Too bad that DL20 G9 doesn't support SR-IOV, it was a good unit for small VM platform and two BCM NIC will be not enough. Add one 10G NIC with SR-IOV vitual NIC was helpful for network performance in it, I don't want to use the Esxi vitual switch function.