I closed the ticket with wontfix, as the dependency on selinux-policy-base is what the RPM macros for packaging selinux generate. This is beyond my control. The packages for EPEL are built against RHEL and first must conform to some reasonable packaging standards and second YMMV when using/installing them on derived distributions, especially ones that out way behind with the updates wrt. RHEL.
To make this work, you can either grab the SRPM and build the package yourself, or use the unofficial repo which carries snapd rebuilt without SELinux, see Unofficial snapd repository for Amazon Linux 2.
That being said, I think the best course of action is to ask Amazon to include snapd in their Extras repository, thus making it readily available. We’ve tried to make it in the past but to no avail. I’m not Amazon’s customer so I can’t even file a support ticket. The snapd upstream builds and runs a suite of tests on Amazon Linux 2 for each pull request, so there’s a high chance that the integration process will be of low effort.