The only bit that I see that comments on Magical Success is the indented section on p161:
The bearer of a Marvellous Artefact or Wondrous Item gains (2d) when making rolls using the Skill corresponding to its Blessing, and allows the Player-hero to achieve a magical success (see page 21).
The problem is that your understanding presupposes that Magical Successes cannot be used independently of the specific Blessing. It is also able to be validly interpreted in the following segments:
The bearer of a Marvellous Artefact or Wondrous Item
[gains (2d) when making rolls using the Skill corresponding to its Blessing,] and [allows the Player-hero to achieve a magical success] (see page 21).
If the generic application of Magical Success was intended, the sentence still works, and there's nowhere (that I've noticed) that unambiguously says you can't. As I said, I'm pretty sure the specific application is intended and I think the game would be worse without the restriction, it's just not unambiguously stated which makes RAW different.