That is not fluff. It is in the rules under the entry for Fused Eidelon.
http://www.d20pfsrd.com/classes/base-classes/summoner/archetypes/paizo---su mmoner-archetypes/synthesist
"A synthesist summons the essence of a powerful outsider to meld with his own being. The synthesist wears the eidolon like translucent, living armor. The eidolon mimics all of the synthesist’s movements, and the synthesist perceives through the eidolon’s senses and speaks through its voice, as the two are now one creature."
Is should be immediately apparent to anyone looking at an Synthesis Eidelon that it is not a true creature, but rather a husk covering the creature inside.