This is likely a typo. The spiderhawk archetype cannot replace the character's level 15 feat gained from character advancement, because class archetypes cannot do that.
First of all, an archetype cannot replace a character's normal feats that they gain via character advancement. Class archetypes are only able to replatce or alter features of the corresponding class. From the explanation of how class archetypes work:
The primary way in which archetypes modify their corresponding base classes is via the use of alternate class features. When a character selects a class, he must normally choose to use the standard class features found in the class's original source—the exception is if he chooses to adopt an archetype. Each alternate class feature presented in an archetype replaces a specific class feature from its parent class.
By contrast, the feat gained at every other level via character advancement is not a "bonus feat". The feat is given according to your character level (total levels in all classes, plus some possible adjustments), rather than your class level (number of levels in a specific class). These feats are not class features, and therefore cannot be replaced by a class archetype.
The spiderhawk archetype can replace any magus class feature, such as the "bonus feats" that the magus gains as class features at 5th, 11th, and 17th level. Since the magus has no 15th level bonus feat to replace, the most logical explanation is that this is a typo, probably intending to replace the 17th level bonus feat as you suggested.
The spiderhawk was written by a third party publisher, and so I cannot say if or when the publishers would publish errata to correct this typo.