Welcome to the Luos community !
Yes, Robus is the transport layer. From an OSI point of view, it represents MAC but it is intricated with some network things allowing us to improve a lot the performances of the lib.
Basically, Robus have some notion of nodes, services, and services type allowing it to make a first filtering and basic routing work on messages.
Robus use LuosHAL : https://github.com/Luos-io/LuosHAL to interact with the physical world this is the PHY layer. This repo has a different target and this is the place where you can define your physical layer.
Generally, Luos users use RS485 or OneWire network, but some people tweaked this HAL and make it work on BLEmesh or other weird things. So you can adapt it to your needs.
From an OSI point of view, things are a little bit blurry on our code. We will try to improve it soon to make it more obvious.