As I mentioned, the Colts had a dry film lubricant that was applied to the inside of the upper receivers. This was sprayed on like a paint and allowed to cure. That is why they are probably so smooth.
I looked at the SI defense website and they just callout that their receivers are black anodized. That would tell me that they are not hard anodized but a Type I or II, class 2 . If you really want to find out, call them and ask them what specification their finish meets. Mil-A-8625 Type III is hard anodizing.
Again you are not comparing apples to apples here. The Colt receivers are milled from a forging and the SI are machined from billets, the Colts are hard anodized and the SI's are probably just type II anodized and the Colt receivers have the dry film lubricant applied to the inside of the upper.