Yammahoper is essentially correct.
The intention of that line is to stop characters from using Spell Adders and such to cast spells that they normally could not cast without ESF. Spells above the caster's level, spells not normally learnable, etc.
Our intention (cannot speak to the intentions of the original developers) was not to prevent their use due to wearing armor, and certainly not when using the Armor by the Piece rules from Combat Companion.
Even when using spell devices, you have to make a roll, thus the mods from armor, helm, and equipment carried would all be applied to the casting roll when using such devices, while ESF mods for the spell being of a higher level than the character would prevent the use of spell device altogether.