As mentioned by lhoffer above the unsupported statement is applicable when you are using cross-VC + F5 service insertion.
Service insertion is used when you are using NSX and third party (such as F5) and integrate their management and control plane for automated deployment
F5 service insertion is uses F5 BIG-IP Virtual Edition (VE) in conjuction with BIG-IQ.
You mentioned that you are using physical appliance, so I believe you are not using service insertion and this limitation/unsupported statement is not applicable in your case
In some cases, the F5 BIG-IP is bridged to the VXLAN logical switch so the VMs attached to VXLAN can be in the same layer 2 network as the physical BIG-IP so the gateway of the VMs is F5.
Please note that Layer 2 Bridging does not work with Universal Logical Switch
L2 Bridges: you cannot add a bridge to a universal logical switch
Not sure if you are referring to a same design guide, but there is a NSX F5 design guide here: NSX F5 Design Guide v1.6.pdf