I couldn't find anywhere on that page that said what the service actually does (outside of vague marketing-speak). However, it's clear that the service relies on you providing it with an awful lot of sensitive data.
They seem to be scanning your email inbox and detecting which companies you have an account with. They they send a data removal request to one of these companies "on your behalf". When a company receives this request, of course it's not going to just delete your data since a request is coming from a third party. So they will either disregard the email, or in the best case they will contact you directly for confirmation.
It seems to me that you have to give up access to all of your email to some startup just to be able to do GDPR requests with extra steps for everyone.