I can understand that if they are selling gigabit fibre, deploying optional modem-only installs, and having inexperienced users complaining they aren't getting gigabit speeds because they are using some €15 mini tplink WiFi AP thing, then having to trouble shoot that and potentially look like the bad guys by saying "the hardware you have bought is trash".
I would get mad about that if I had to support that.
Maintaining some sort of "minimum requirements" or "only supported hardware" list considering there are SSSOOOO many routers out there (never mind whitebox openwrt/opnsense/pfsense/mikrotik/raw-linux whatever) is impossible.
And Intel pcie 4-port gigabit cards are so commonly counterfeits (especially on eBay, for those enthusiasts) which could throw so many issues before it even gets to os/userland.

I don't know what the Router Freedom thing is. Sounds amazing to me based on the name (and knowing the EU).
I can understand why ISPs might get antsy when stuff has to run gigabit wirespeed.
But maybe "we only guarantee wirespeed on our hardware"... but if the hardware they are providing is just a modem, it might be hard to remotely debug and provide support.
I think I'm rambling.