Um, no, other than arguably being wrong on the broad kind of motivation in either case, you seem to have also confused the word “cisgender” (or “cis” for short) with the word “heterosexual”.

Transgender people can be heterosexual or homosexual or asexual or any other sexuality, and the same is true of cisgender people; having a term to refer to people who do not deviate from the until-recently-in-our-society culturally obligatory stereotypical relation of gender identity and sex characteristics at birth does nothing to reduce heteronormativity of language compared to just referring to them as “normal” (and then, depending on context, elaborating further on the axis of normality and possibly whose norms are being referenced), to the extent that having a word does that, its the word for people who don’t deviate from the until-recently-in-our-society culturally obligatory stereotype of sexual preference for partner gender based on their own gender.

But, independent of undermining the norms involved, its sometimes useful to be have a convenient concise term, which both “cisgender” and “heterosexual”, in their respective domains, provide.