Even if they could get a source, I wouldn't trust TP-Link. Their entire business models revolves around making routers as shoddily as possible, of course they have a vested interest to block open source firmwares on cheaper models from eating up their fake premium models.
I was skeptical. Type "router firmware" into google news.
Many articles about firmware lockdown
Free router firmware and free software aren't the same.
Although I would note firmware is software. But yes, they are not banning Free Software. This is more an issue with making the routers into treacherous computing platforms [1].
The radio firmware is usually included in the OS. Thus if you want to change the OS on a router, phone, tablet, whatever else and that is the case you won't be able to do it at all.