Too much dimming to be a transit. Betelgeuse is so large that we can map its "surface"[0] and even planets in our own solar system are so small[1] that something so large to cause such a substantial dimming would have been detected a long time ago.
[1] Jupiter is about the largest a planet can be, in the current conditions of our solar system -- that means Jupiter was larger when it was younger, but it's been cooling off and reducing its volume
