Soulmate does not necessarily mean being involved in a romantic way. As such, anyone could be one's soulmate. Your brother can be your soulmate or your best friend etc.

Then again with the intensity and/or intimacy created within the therapy context, one can (easily) mis-perceive the relationship as something it is not. A therapist may be doing their job showing care, giving their undivided attention etc. towards their clients vs. a client believing there is something more... when there isn't.