Can a monster cast a higher level spell using a lower level spell slot? (Shadow Fey Enchantress)

Is there a (monster creation) rule that allows a higher level spell to be cast using a lower level? I ask because I came across the "Shadow Fey Enchantress" (Kobold Press "Tome of Beasts", pages 170-172), as it states:

Spellcasting. The shadow fey is a 10th-level spellcaster. Her spellcasting ability is Charisma (save DC 15, +7 to hit with spell attacks). She knows the following bard spells.

Cantrips (at will): blade ward, friends, message, vicious mockery
1st level (4 slots): bane, charm person, faerie fire
2nd level (3 slots): enthrall, hold person
3rd level (3 slots): conjure fey, fear, hypnotic pattern
4th level (3 slots): confusion, greater invisibility, phantasmal killer
5th level (2 slots): animate objects, dominate person, hold monster

However, Conjure Fey is a level 6 spell: https://www.dndbeyond.com/spells/conjure-fey

Shadow Fey Enchantress, stats: https://www.5esrd.com/gamemastering/monsters-foes/monsters-by-type/humanoids/shadow-fey/shadow-fey-enchantress-3pp/

I checked with the Errata, but it doesn’t mention this as an error. Perhpaps it was overlooked, or perhaps there is a monster creation mechanic that I’m not aware of?

EDIT: Bonus points for a suitable replacement spell (if indeed an error)!