It generally makes more sense to trust CPU specifications and utilize it fully, than to assume everything is tampered with and only ever use `mov`

If they don't use it, the kernel does in order to initialize the entropy pool, which will also result in broken boots. To work around historically flaky kernel implementations, systemd calls RDRAND directly if it detects it needs to with a heuristic. This is why the fix for the current issue is to set both 'nordrand' and 'SYSTEMD_RDRAND=0'.

What happened to it's not systemd's job to fix kernel bugs?

I didn't type that, so I'm not sure what you're referring to. systemd is fundamentally tied to Linux and thus needs to implement workarounds from time-to-time when kernel facilities are lacking. This is complicated by the requirement that systemd continue working on older Linux versions.