(from here: https://www.dndbeyond.com/compendium/rules/basic-rules/spellcasting#KnownandPreparedSpells )
These features that let you use an ability without a spell slot are an exception to this general rule- they say you can cast the spell a limited number of times through the trait, but do not also say the spell counts as prepared/known for you, so you cannot expend spell slots on them. You can just cast them through the trait.