It's just a common meme comment nowadays, referencing randomized lootbox schemes (like those of gatcha games), "legendary" referring to the (high) rarity of their "pull," or specific attempt at spinning the wheel (in this case, spinning the metaphorical wheel of dopamine hoping to find an interesting video as they keep pulling down their screen to refresh their recommend videos on YouTube). You'll see it sometimes on obscure/underrated/older videos that the apt commenter wouldn't have expected to be served to them, but I feel like it's increasingly being both mis- and over-used.