Skill based matchmaking is in the game. Clearance level is not the same as skill.

Does Rainbow Six siege have skill based matchmaking casual?

Casual and unranked, yes. It's all got hidden MMR so that the more you play of it the more you're matched with other players of your skill.

Does Rainbow Six siege have SBMM?

Like Call of Duty, Rainbow Six Siege has a form of SBMM in its Casual mode — mostly designed to make sure you don't sit there crushing brand new players every time you play. ... Over time, you get games against players who are roughly around your same skill level.