Like, perhaps this problem is also affecting other kinds of usage, but the original article does not attempt to claim that, and purely from their example it would be generous to assume that literally dozens of individuals would need this feature and, well, it's not worth to make and test features (even if they're just a configuration option) in this case.
I suspect the concern is not that ordinary users would be targets, but that ordinary users would be sources of ddoses (by being part of botnet)
In general it's extremely unlikely unless you are engaging in "high risk behavior," but at the scale of an ISP there are enough users doing that kind of thing (Twitch streaming, etc) that it becomes an appreciable frustration for your network operations.
This sounds like the sort of thing with similar prevalence to things like running a Tor node. This might even be an example the other way, when your game server or what have you has thousands of peer connections and this thing breaks it by misinterprets that as a denial of service too.
If they say they have no idea what you're talking about, you get to tell them they're infected, so they actually fix it instead of typing their bank password into the infected box the next week because you automatically removed the "huh, internet's slow" that might have led them to investigate it otherwise.
Which is all just one reason that, of the set of people running Tor relays on residential internet connections, I'd wager a solid 99% shouldn't be.
Furthermore, you need some kind of ONT for fiber termination and it isn't clear that Centurylink uses a different ONT without this feature for business class customers.