This is not a forum with legal advises. I inform people about an option, which they asked for. GNU/Linux phones have a similar security approach to GNU/Linux on desktop. People explicitly seeking GNU/Linux should know this. They can also ask or search the Internet.

> I think it’s irresponsible to promote it as an alternative device without noting that it’s less secure and full of footguns

I disagree with you here. Informing about options is better than not informing. "Less secure" depends on a threat model. GNU/Linux on desktop is working well enough for millions of people. So it is a viable security approach for many. Saying that your threat model is the only one that should exist and be promoted is crazy.

> only fits that definition under FSF technicalities

This is one of the strictest definitions there is. By which definition does GrapheneOS run FLOSS?

> same set of proprietary hardware, just with different communication mechanisms/boundaries

More choice is always good, isn't it? If it is not for you, you are free to use and promote the duopoly. (Yes, I consider AOSP obeying Google's development strategy long term. It will not end well. See: this topic.)