Seems more a reference to OpenWRT who as the name suggests will only use stuff that is open source.
Alternative custom firmware developers don't have such an issue.
I don't think that's true as such, the modem firmwares aren't open source and they use them and they HAVE to use the open source drivers for things like Atheros WiFi because they have a different branch of firmware for the open source driver only provide the closed-source to hardware manufacturers.
Another problem is when there are custom patches to the kernel for specific hardware, they need to keep it as generic as possible to support such a wide variety of hardware.
There are legal ramifications too I think, where you down want to risk the whole project for doing something that might not be 100% in line with what the manufacturer allows.
Fact is, not all hardware vendors will provide enough source code to actually compile a working firmware and even if they do it will be a very specific build environment that is not compatible with OpenWRT. Broadcom for example is often impossible to support, DD-WRT have a special deal with some vendors to be able to support it on specific routers but AFAIK that requires a LOT more work which holds back support for other hardware and newer software versions.