Yes, a portal is an object, BUT, the data in the portal is still "join" data (data being the keyword, as opposed to object). Refreshing the window with the "flush join data" checkbox might be the expected action still.
I think all the refresh object script step is more geared towards showing/hiding an object with the new "Hide Object When" calculation.
Reading into it, this was found:
If the specified object is in a portal, Refresh Object updates the object in every visible portal row.
Again, using the term "object" and not referring to the data at all.
The help also says that Refesh Object refreshes the content of the object. I read this as the actual data content. The help is therefore slightly misleading and should probably be clarified.
And actually, just setting the portal field to itself with the set field step will actually cause the portal to refresh, even without the flush.
I also tried refreshing the field in the portal, but that did not work either.