I think he means that if engineers cost X in resources, and you're using them on a space dock that is not getting any benefit from their being there, that that's a waste of the resources of the engineer.
The answer is kind of complex; it depends on what you are producing. If the thing you are producing is very expensive and has a low rate of build/repair, like starships or golems or spirecraft, then you can put dozens of engineers on there and all of them will be working at peak efficiency.
However, the maximum output from a space dock is 1 ship for every two game turns, so at the default network skip settings that's 1 ship for every 400ish milliseconds. So, 2.2 ships per second.
If you stack engineers beyond that threshold for a ship that is cheap and quick to produce, then the extra engineers aren't doing anything and the space dock winds up waiting for part of each 400ms cycle, with the engineers still costing you energy but not costing you actual construction costs (same with the space dock) since they are just sitting there. Generally with most ships, 2-3 engineers on a space dock is not a waste, although it will be overkill every time something like Fighter Is come up in the queue. But only a waste briefly, and in terms of energy of the engineers not being ideally distributed for that brief time.
