"No “free” streaming service is worth the risk of pissing off the Xfinity limit police and losing my Internet service."
this is only a true statement if the value of your current service is infinite or your risk aversion is. to make any kind of realistic logical statement about this you need to calculate how much bandwidth is being used, user limits and isp switching costs.
a nice insight turned into fear mongering - this is how a lot of bad memes start.
Considering how in a surprising number of places there is only one broadband ISP available, it's not exactly unreasonable - though I'll agree it's hyperbolic - to say that the value of his current service is 'infinite', since internet access - at least in the developed world - is fairly important. Have you used dial-up internet lately? It's not really a fun experience. (n.b. I've not used dial-up in a while, but my mother has a bad DSL line that tests at around 0.2Mbps, and I was not amused by it the last time I was there. This is my basis of comparison.)
If spotify is silently operating a P2P client, even when it's not being directly used, then that's an issue for people with bandwidth restrictions. It's trivial to get distracted by something, forget an application is running when you leave the house, and end up using much more bandwidth than you intended. It's worse still that as bandwidth increases, usage limits don't stay proportional. You suddenly have the ability to share faster, reaching your limit sooner.
As I said, I agree that the statement is hyperbolic, but I don't consider the issue trivial.
I don't think you are in control of how much bandwidth is being used when you invite someone else's software to use your machine as a server. The streaming service could change at any time and run your connection full speed for an indefinite period.