If I were to guess I would say it is bleach that is actually mixed WAY stronger than necessary for cleaning. Any stripper will darken the wood, not lighten it. Precarbonate bleach usually leave an inconsistent look especially when dry. Looks like good old chlorine bleach. Anybody spraying that strong a mixture near grass or anything will be pissed. If these guys were so smart they wood have a roof cleaning set up in there truck and knock out the fence in minutes. I always brighten after using bleach, bleach is a base just like a stripper just not as high up on the alkaline chart. There are other reasons for using a brightener, especially oxalic acid. One, it not only neutralizes wood tannins but also iron, rust, and metal stains in wood. On fences this make a HUGE difference in a quality job by removing these stains. Secondly, it works quickly and you get a good idea how the end result will turn out and how well the surface will look. Third, oxalic acid helps open the pores of the wood helping with penetration of your coating.
