Resend seems to be a UI layer on top of SES, so it is probably not a competitor. People could use SES directly, but if the target group for Resend is a “modern front-end dev” then they probably do not want to, or do not know how to, and thus need that UI layer.

For example, I maintain a popular email sending library (Nodemailer) and if I could get a penny any time some user files an issue with something that is clearly basic for anyone who has ever even touched email, I would be quite wealthy. All these people are good match for a service like Resend, they just want to send mail for a reasonable price and do not care the slightest of what is going on under the hood.