The author seems to be saying that von must be used when there is no determiner present to take the genitive case. In the first example we see a null-article, which obviously can't take a case. Here I would agree that you really must use the von-construction.
genitive with a declinable determiner: der Bau der Kraftwerke; der Bau eines Kraftwerks; der Bau schöner Kraftwerke
von-construction whith an indeclinable determiner: der Bau von [null-article] Kraftwerken; der Bau von fünf Kraftwerken
But when we look at that final example you were having troubles with, we can actually use multiple varieties:
genitive with a declinable determiner: ein Strahl der Hoffnung
von-construction whith an undeclinable determiner: ein Strahl von [null-article] Hoffnung
partitive apposition with a null-article: ein Strahl [null-article] Hoffnung