I am currently struggling to understand this as well. I have an identical ArcGIS Enterprise setup only I am on 10.6 . I have been able to upload an albeit smaller SLPK file through my Portal and I have also used the Share Package tool. Both times I have no option to publish it as a layer. When have loaded the same SLPK into my organization's ArcGIS Online instance and there is an option to publish the SLPK as a hosted layer right from the Add Item dialog box (see image below) This is frustrating. Hopefully Esri will ensure that this capability exists in Portal as well.
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