Xfinity Stream Beta, I am confused

So, i am an xfinity internet only customer, and last week someone told me that they were able to use a Roku in place of a set top box for xfinity cable. I do not have cable, but my parents have xfinity cable and internet, and phone, and I thought this might save them the box rentals each month. So I googled it, and found vague and sometimes contradictory information. I logged into my xfinity account on my computer, and found I was able to watch what seems to be all xfinity’s TV channels online. I also put the Xfinity Stream Beta app on my roku, just to see how to set it up, an found that I can watch xfinity’s full channel lineup on it.

The only thing I can find online is that they are testing a streaming service called xfinity instant that will be $20 a month, and here:Comcast will charge extra fee for watching TV on Roku boxes | Ars Technica it says it is currently free (for testing I assume), but is limited to customers who rent a set-top box (I do not. I have internet only).

And the article linked above says using a roku will save you a whopping $2.5 a month (the set-box rental is $10 a month).

So, i’m really just trying to figure out exactly why I seem to have access to the full TV lineup (through my roku) even though I am not paying for cable.