Using the Play Store on GrapheneOS whether via sandboxed Google Play or another frontend definitely doesn't defeat any the purpose of it. You do not have to use the Play Store on GrapheneOS, but the privacy and security features it provides are not cancelled out by using it.
GrapheneOS has privacy features such as Contact Scopes and Storage Scopes which are most useful when using privacy invasive apps. Using privacy invasive apps doesn't defeat the point but protecting against those is a core part of the purpose of GrapheneOS. Our Sandboxed Google Play compatibility layer is a privacy feature itself to enable people to use those as regular sandboxed apps without invasive access to be able to run apps depending on them.
GrapheneOS has privacy features such as Contact Scopes and Storage Scopes which are most useful when using privacy invasive apps. Using privacy invasive apps doesn't defeat the point but protecting against those is a core part of the purpose of GrapheneOS. Our Sandboxed Google Play compatibility layer is a privacy feature itself to enable people to use those as regular sandboxed apps without invasive access to be able to run apps depending on them.