He makes the point that they generally argue from what could be called an "It's not a blue sky" principle

Lindsay provides this example:

Suppose someone says, "The sky is blue."

A social justice warrior response would be,  "Well, it is not at night."

The SJW response is technically true but it misses the point. 

The first statement was clearly only referencing the daytime and color of the sky at that time, so attacking the point by arguing that it is not at night is just dragging the argument in a very unproductive manner in an attempt to imply the statement is wrong.

But in speaking (and writing), we always skip all kinds of clauses and subpoints, assuming they are understood.

To be completely accurate, one would have to say:

 The sky is blue when looking up from the planet earth during daylight hours and not when it is rainning or overcast. Further one can not see the sky when looking up while on the planet earth if one is looking up from inside a building or other overhead structure or if the trees are extraordinarily thick above.