We've had this conversation before.
[url]AM5 Book binding]
It would probably make a good entry on the "suggested errata" thread.
The short version is, yes, that's what ArM5 says, but the core book isn't consistent with published uses of environmental trigger in the 5th edition supplements, which seem to allow environmental triggers to be used far more broadly. (Consider the "Fire Guardian" in the Semita Errabunda write-up and "The Crystal of a Hundred Candles" from Guardians of the Forest.)
I anticipated your objection, though.
Returning to the subject at hand, however, I like the idea of putting the weapon-ward spell into a magus' talisman. If the object triggers off the approach of weapons (detected by an InTe spell, if necessary), you can make sure that the magus is completely warded (so no Achilles' face) during only those times when the magus needs to be completely warded (so no Warping).