Does a Necklace of Fireballs max out at 9th level casting?

This question arises because the section on Spell Level in the PHB states “Every spell has a level from 0 to 9”, but the Necklace of Fireballs can have up to 9 fireballs on it, and it says “You can hurl multiple beads, or even the whole necklace, as one action. When you do so, increase the level of the fireball by 1 for each bead beyond the first”…meaning a full Necklace of 9 fireballs thrown at once would equate to an 11th-level casting of Fireball.

Is that even legal?