This is such a facile meme, and now you've stretched it to a new level of absurdity. Lawyers doing pro bono work don't have clients? Somebody for whom you are doing work for trade isn't a client?
Nonsense. If someone is using your property (or widget) without your permission (or license), it is reasonable to consider them a thief (or pirate), and absurd to consider them a client (or customer).
That description is almost but not quite entirely unlike what actually happened. They were using the widget with permission, and suddenly they changed their minds and wanted to start charging for it, but without doing anything to enforce that.
It may or may not be unlike what happened depending on if, when and how the terms of service were revised.
For example if the TOS were revised two years ago, then it is possible that there was no longer permission to use it (it is also possible that permission continued).
If you are not cashing someone's checks, they are not your client.