Hilarious conclusion from the author. It's almost certainly not the case that the owners of this service are using it to 'DDoS' targets, rather it's much more likely they are using your device to host a proxy server and then selling access to some 'residential proxy reseller'.

On the other side of that, some random Joe has probably purchased access to a set of these 'residential proxies' and is using them to scrape flight data from the airline site the article author noticed, with some of those requests being sent over the author's connection.

Many 'free vpn' and 'free proxy' apps engage in this behavior, you may proxy your requests via their connection, but they also proxy their requests via yours, generally reselling that access to someone who finds your IP address to be of value to them due to the fact that it's not a datacenter address.

It's certainly questionable to straight up unethical either way, especially so if the service doesn't disclose to you that they're doing that, but on the other hand I find the author's DDoS conclusion to be so contrived and out of touch with reality that I had to write this comment.