Yes, we do have an A2 lesson that says that we should use estar with civil statuses and this is indeed the general usage. We don't go into depth in this lesson about certain nuances because this would be more advance, but we do use ser sometimes with civil statutes; generally this is a more formal usage, for example, when filling up forms, there may be a question saying ¿Es usted soltero/-a?, or if we want to say that we are single seeing this status as our social identity, we may also use "soy soltera" considering this as a more permanent status, maybe thinking that I may stay single for a long time or forever, with no intention to change that status. This is the case in the sentence you found in this lesson.
As I said, the A2 lesson doesn't explain nuances, it simply states what the general use is.
I hope this clarified it.