You should carefully read: “… when using an external chip such as the MCP2518”.
This surely is true because the MCP2518 manufacturer has licensed its usage.
In my view it would likely not be true for things like GitHub - KevinOConnor/can2040: Software CAN bus implementation for rp2040 micro-controllers and I do not know how it would work for all the rest that might be somewhere in the middle. Once again, I’m not the expert in licensing stuff, nor do I intend to become one.