gitto - 'Letters at the imagined world's corners...' means we have the imagined world in the grid, and its corners give a synonym of the 'force'. Obviously we can't shade the whole synonym, as it would disrupt the 'world' - we can't treat the corners in the same way as everything else because they're single letters, so a simple 'shade' instruction is impossible. However, by telling us that the corners are part of the world we can deduce that they must be congruent elements.
If you haven't highlighted the relevant corners you haven't revealed the world - it's a particular thing with a particular shape, and we know what occurs on it because we have the encircled letters (they are very specific elements). Why destroy the meaning of the quotation - which relies on the imagery in the grid - simply because an overt 'shade' command isn't given? It would be extremely clumsy to attempt to fashion such an instruction and completely unnecessary. We know what we need to see in the grid, anything short of this is a travesty.