The problem with that logic is that spellcasting is called out as a Special Attack. Special Attacks are, by RAW, one of Ex, Su, or Sp. They cannot be Na.

See p. 6 in the MM: Special Attacks and Special Qualities. This section explicitly breaks Special Abilities into these two groups. This same section also explicitly states that Special Abilities are one of Ex, Su, or Sp.

On p. 299-300 of the MM, "Spells" is explicitly listed under Special Attacks.

As "Spells" is a special attack, and special attacks are special abilities, and special abilities can only be one of Ex, Su, or Sp (they cannot be Na), then Spells must be one of these three.

Su are stated to be magical in nature, are not spells (MM 315, Special Abilities). Sp are also explicitly stated to not be spells on the same page. By simple process of elimination, Spells MUST be Ex because Na are not Special Abilities at all.