And this is the required means, that the router vendors prevent loading of software that does not meet the desired ends. Previous documents instead specified that DD-WRT not be loaded.

The FCC document is no longer available: please see http://web.archive.org/web/20150803065407/https://apps.fcc.gov/kdb/GetAttachment.html?id=1UiSJRK869RsyQddPi5hpw%3D%3D&desc=594280%20D02%20U-NII%20Device%20Security%20v01r02&tracking_number=39498

It is cited in https://via.hypothes.is/http://www.wired.com/2015/09/hey-fcc-dont-lock-wi-fi-routers/

This, however, changes the discussion to make the user or operator, rather than a third party, unable to operate the router outside of the legal limits.

This is the beginning of the discussion of interest in the FCC's proposal which would ban open source operating systems.

It should be noted that this is not proposed by the FCC as a ban as such, but rather an effort to keep the devices in question operating within the law and regulations.

Our concern is it changes the regulations to make it difficult or impossible to use open source or free software operating systems, and that initially it named DD-WRT as an operating system that vendors were to required to prevent from use.

The document mentioning DD-WRT has subsequently been amended, For the relevant copy, see the Internet Archive at http://web.archive.org/web/20150803065407/https://apps.fcc.gov/kdb/GetAttachment.html?id=1UiSJRK869RsyQddPi5hpw%3D%3D&desc=594280%20D02%20U-NII%20Device%20Security%20v01r02&tracking_number=39498 at the bottom of page 2

The common sense of "third party" is a person other than the vendor and purchaser of some thing.

The citation of DD-WRT The linked document is no longer available at the FCC link address.