This is my reaction also. You got a great acceptance and a great opportunity at Cal Poly.
I do not see anything in your original post that would change UCLA’s decision.
You can do very well with a degree in CS from Cal Poly.
When I was applying to graduate programs, I was turned down by my “dream school”, went to my second choice, and LOVED IT. I think that you are likely to have the exact same result here (except for your bachelor’s degree). Plan to work hard at Cal Poly, keep ahead in your class work, keep in mind that software can take longer to debug than you expect, and I think that you will most likely do great.