Netgear: "If a third party firmware is used, warranty will be void."

I am the owner of a Netgear R7000 router and since Netgear writes on the page http://www.netgear.com/home/products/networking/wifi-routers/R7000.aspx:
I assumed, that it's no problem to install third party firmware, and that Netgear somehow supports it.

Because the driver of the...

Netgear: "If a third party firmware is used, warranty will be void."