How do Bolts From the Grave and a Rakshasa’s Limited Magical Immunity interact?


We recently included an UA Revived Rogue subclass in the group and the players had to face off against a Rakshasa. The question is, Bolt From the Grave says it’s a Ranged Spell Attack, but doesn’t get cast with a spell slot.

Bolts from the Grave. At 3rd level, you have learned to unleash bolts of necrotic energy from within your revived body. Immediately after you use your Cunning Action, you can make a ranged spell attack against a creature within 30 feet of you, provided you haven’t used your Sneak Attack this turn. You are proficient with it, and you add your Dexterity modifier to its attack and damage rolls. A creature hit by this attack takes necrotic damage equal to your Sneak Attack. This uses your Sneak Attack for the turn.

While the Rakshasa has the following:

Limited Magic Immunity. The rakshasa can’t be affected or detected by spells of 6th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

Previously I had decided that the Paladin ability to detect fiends, undead etc would work to find the Rakshasa (he has an amulet to hide his aura), so I feel like Bolts from the Grave should be effective. I was just wondering what the general thoughts would be on a ruling.