No, cisgender is not a subset of a person's true sex/gender. Cisgender is a term used to describe people whose gender identity aligns with the sex they were assigned at birth. So, a cisgender man is someone who was assigned male at birth and identifies as male, and a cisgender woman is someone who was assigned female at birth and identifies as female.
A person's true sex/gender is their internal sense of their own gender identity. This is not always the same as the sex they were assigned at birth. For example, a transgender woman is someone who was assigned male at birth but identifies as female. Their true sex/gender is female, even though their assigned sex is male.
It is important to respect people's pronouns and gender identity, regardless of whether they are cisgender or transgender. Using the correct pronouns and gender identity can help to affirm people's sense of self and make them feel more comfortable and respected.