There are several abilities in 5e that allow users to go invisible without casting a spell or using a magic item. Examples of which are: Empty Body (monk), Channel Divinity: Cloak of Shadows (Way of Shadow Monk), Cloak of Shadows (Trickery Cleric), Umbral Sight (Gloom Stalker Ranger), and Misty Escape (Warlock). None of these explicitly say 'magically invisible.'
Should I say that all forms of invisibility are magical when I am a Dungeon Master even though the book says otherwise?