Thanks for sharing that link, very helpful to know! One less thing to include in a custom image! We have installed Auto Deploy 5.1, and yes you're right in that when I look at a host provisioned via Auto Deploy, it does have the vmware-fdm VIB installed. I incorrectly assumed it was getting installed during boot in stateless mode as the host profile was being applied and the host added to vCenter. But the information you shared makes sense.
Now makes me wonder what happens if you already include vmware-fdm in your image when Auto Deploy 5.1 receives the new image? I would guess it either overwrites what's in there or skips the inclusion. Either way, if you get it by default via Auto Deploy, no point worrying about it via Image Builder. That actually works much better because in the future if you have multiple vCenter/Auto Deploy installations, as you upgrade environments, your ESXi image can remain the same.