Given all the limitations this API has, including "Emails can be scheduled up to 72 hours in advance.", the use-cases they mention like scheduling an email for an event or a weekly digest, will still require using a Job Queue in your application (like Oban for Elixir), mostly negating the managed-service aspect here.
Another issue is, when using a job queue, you may check at the time of sending the email whether you still want to send it. With queuing it, you'll have to periodically check for all scheduled emails whether you want to cancel any.