Essentially yes, the video block is just an easy way to either work with the Vimeo integration, or to stick a play button on the thumbnail of a linked video's thumbnail. If for whatever reason the separate system we use for pulling that video's link info (including thumbnail) is unable to get that info, the image link function is the default workaround to make sure your video still gets linked in a visual way.
In the past I've also used third party sites to be able to just upload an image, and have that site stick on a particular style of play button that's more in line with the visuals I'm going for. I mention this in case you're wanting to make sure the fact that the image is clickable is especially highlighted.
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