Appimages came before these tools, and the tools (forgot the name GearLever, AppimagePool is another one) came afterwards.
They are structurally better as they are external.
That verification is interesting. So it is another appimage, used to verify appimages? Are all Appimages using that, if not what percentage of the ones you know? And are tools like Gearlever enforcing or using that signature check?