I agree with Rusty. The impersonal expression "tratarse de + complement" is synonymous with "ser".
- Las aves no deben ser mascotas. Se trata de seres delicados que requieren libertad para volar.
The birds should not be pets. They're delicate beings who need their freedom to fly.
- La guerra no se resuelve en dos días. Se trata de problemas complejos que no tienen solución fácil.
War can't be ended in two days. These are complex problems that don't have an easy solution.
- No veo por qué enojarse. Sólo se trata de una opinión.
I don't see why getting mad. It's just an opinion.
The author is not using "estaba segura de que era él", because in writing, usually less elemental forms of language are preferred.
