Hi Mark,
> ... is a Map Image Layer just what ESRI refers to as a hosted map service
No. The term "map image layer" is a portal item type. It could be powered behind-the-scenes by either a hosted map service, or a map service from an ArcGIS Server site (e.g., federated or registered).
> I was expecting just a "Map Layer" to be added to Contents in Portal ...
Yes, when you share content from ArcGIS Pro to a portal (either ArcGIS Online or Portal for ArcGIS), you end up creating a new portal item. Sharing a data layer in your ArcGIS Pro map, will create a new "map image layer" portal item - assuming you did not enable feature service. Depending on how you shared the layer, the data could be referenced or copied to portal.
> ... if Map Image Layer and Map Layer are synonymous then I don't have an issue.
I don't think the term "map layer" is really official in the ArcGIS help documentation. But I know sometimes users use the shortcut term "map layer" instead of the official term "map image layer" to refer to the portal item type.
Hope this helps,