I think this line added the default site - https://github.com/roots/trellis/blob/b7ecfeb8590a46283f231e8606bf2e05ea20c3f6/roles/wordpress-setup/tasks/self-signed-certificate.yml#L21
Self-signed certificates not being generated in development sees the syntax error as well.
Probably coming from https://github.com/roots/trellis/blob/b7ecfeb8590a46283f231e8606bf2e05ea20c3f6/roles/wordpress-setup/tasks/self-signed-certificate.yml#L3-L16