The goal of nearly every site or app is to get users to engage as quickly as possible. That’s why it’s surprising that so many sites have a separate flow for registration versus sign in. There’s no good reason that these two flows shouldn’t be combined.
A part of nearly every registration is entering an email and password so why not make them the first step of the sign up process? If that email and password are already in the system just log her in. And if not then you have the email address of a to-be-registered user. The only real risk is if someone thought they had an account and wanted to sign in but instead signed up - and that’s a good problem to have from the application side since you have the contact info of an interested user.
I’m impressed whenever I encounter a site that does this since it’s so rare - it’s not a complicated change and while not impactful it is a nice and easy way of reducing friction and making everyone a tad more efficient.