Can trans feminine people use the contraceptive pill in place of hormone therapy?

This is not advised. The oestrogen hormones in contraceptive pills are in a different form, and at different levels than those used in hormone therapy for trans women and non-binary people.

Importantly, contraceptive pills also do not contain the anti-androgens which are used to block the effect of testosterone. Contraceptive pills therefore can’t be used in place of gender-affirming hormone therapy.

If you're taking oestrogen hormone therapy, and have not undergone surgery, you may have reduced fertility, but sperm can still develop in the testicles. So you will need to consider an additional contraceptive method where there is a risk of pregnancy that you want to avoid.

For more info, read this FSRH statement.