Building out clean energy infrastructure provides value. That's perfectly fine. There is plenty of need for hands to maintain American road infrastructure that is falling apart and build new infrastructure like high-speed intercity rail and more subway lines to help support additional population growth.

The reference is to how a government can pay people to literally dig ditches then refill them. This nominally increases GDP (due to additional government expenditure) but it does not produce value and, more importantly, does not give people the dignity they would ordinarily achieve through their labor because they know such work is bullshit.