I actually think it would be unethical to continue treatment with a client the therapist isn't comfortable with, or doesn't feel competent enough to treat, or similar. It would have a negative effect on the client, and therefore the responsible thing to do is to refer the client to someone else. It is in the best interests of both parties, which is the point that is made in your quote, ATAT.

I recognise that this is a difficult topic, though, because if the client experiences it as abandonment that is very much a problem. But if the alternative is to continue treatment and possibly inflict even more harm that way, what is the therapist supposed to do?