That's because it is in the Complete Adventurer (which also features the Daggerspell Shaper for rogue/druid characters).
Edit: I saw you found it yourself while I was typing all this out.
Daggerspell Mage:
Alignment: Any nonevil.
Skills: Concentration 8 ranks.
Feats: Weapon Focus (Dagger), Two-Weapon Fighting
Special: Arcane caster level 5th, sneak attack +1d6.
Average BAB, good Ref and Will.
1st level: Daggercast (can deliver touch attack spells with melee dagger attacks, can cast spells with somatic and material components while holding a dagger in each hand)
2nd to 10th level: +1 arcane spellcasting level
2nd: Invocation of the knife (can make slashing damage with energy damage spells)
3rd, 6th, and 9th: sneak attack +1d6
(Other abilities are gained at level 5, 6, 8, and 10.)
So, an arcanist 5/rogue 1 can qualify for the class. Then, once he's arcanist 5/rogue 1/daggerspell mage 3, he qualifies for arcane trickster. And he only lost two spellcasting levels, the one as a rogue and the first of daggerspell mage. An arcanist/rogue/arcane trickster build would have lost three (three levels of rogue).