All of these are made worse by a fork.

[Edit:] Since I am getting downvoted let me get through their stated reasons:

    Number of active core developers at an all time low, 
    no process for getting more new people involved.

A fork is cutting the number of available devs in half and creates uncertainty for new people. So it should hurt in the short term.

    Unreliable infrastructure, fixes prevented by
    internal disagreements and single points of failure.

Then fix the infrastructure. ( If the fixes are actually prevented by internal disagreement, this may be a argument for a fork.)

    Lack of communication, transparency and coordination 
    in the OpenWrt project, both inside the core team 
    and between the core team and the rest of the 
    community.

This may or may not be improved by a fork. It is certainly easier to manage a community half the size and a fork may be a fresh start, but it seems a better idea to try to improve their process.

    Not enough people with commit access to handle
    the incoming flow of patches, too little 
    attention to testing and regular builds.

Not enough people, so we are splitting the team.

    Lack of focus on stability and documentation.

To some extend documentation and testing profit from enough available manpower.

Apart from the "internal disagreement" argument all of these arguments may be pain points but are not really arguments for forking the project, some are orthogonal some are actually arguments against a fork. So I wish them the best, but from the outside this fork does not look promising.