Why do you think it is not a spell, does it is not use spell in the keyword. For the record, I think that these "spells" given with out a level are not counterspellable but I would count them as spell for the purpose of mage slayer or the paladin aura.If it's a melee or ranged spell attack, it still doesn't mean it's a spell. Some things use that wording without being spells, like a Star Druid's Archer form.
My reasoning is that as they have like innate spells they have no material or somatic components and so the counter spelling caster has no warning and nothing to react to.