Is there a way to build Service Definition files in Pro? I assume not since they have everything on lockdown to Portal. Thought I would ask here though.
You can build .SD (service definition files) in ArcGIS Pro. Only using Python, no "GUI" way to do it.
See CreateWebLayerSDDraft command.
Following the building of the SDDraft and Staging into a SD, you'd use the Upload Tool using the proper key word (My Hosted Services) to share the .SD file to your portal that you've authenticated too.
ArcMap publishes data to ArcGIS Server to create web services. ArcGIS Pro shares content to a portal (ArcGIS Online or Portal for ArcGIS) and can create a web map, web scene, web layers, and their associated web services.
Therefore you would need to use this functionality to publish from ArcGIS Pro.