I wrote an email to them. Here is what I wrote:
Hello, I want to request Smart-ID compatibility with GrapheneOS. GrapheneOS is a private and secure mobile operating system. Developed as a non-profit open-source project.
Smart-ID doesn't work on GrapheneOS because it's not Google certified even tho it has superior security and privacy over stock operating systems that come with most Android devices.
Devices launched with Android 8 or later have hardware attestation support which cannot be bypassed without leaked keys or serious vulnerabilities.
The hardware attestation feature is part of the Android Open Source Project and is fully supported by GrapheneOS. SafetyNet attestation chooses to use it to enforce using Google-certified operating systems. However, app developers can use it directly and permit other properly signed operating systems upholding the security model. GrapheneOS has a detailed guide for app developers on how to support GrapheneOS with the hardware attestation API. Direct use of the hardware attestation API provides much higher assurance than using SafetyNet so these apps have nothing to lose by using a more meaningful API and supporting a more secure OS.
Apps should support GrapheneOS rather than locking users into the stock OS without a valid security reason. GrapheneOS not only upholds the app security model but substantially reinforces it, so it cannot be justified with reasoning based on security, anti-fraud, etc. The guide on how to do so can be found here: https://grapheneos.org/articles/attestation-compatibility-guide