Making arcpy.ApplySymbologyFromLayer_management() work on CURRENT project of ArcGIS Pro

To test this in ArcGIS Pro 2.5.0 I used a project with one map and one polygon layer in it.

1. I opened the project and gave the layer a very bright color, which was easy to distinguish from its original, and used Share | Layer File to save C:Tempreferencelayer.lyrx (I did not have permissions to save it as C:referencelayer.lyrx)
2. I closed the project without saving.
3. I restarted ArcGIS Pro with the same project and saw that the layer had its original colour
4. I opened the Python window and typed in these commands in this order.

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aprx = arcpy.mp.ArcGISProject("CURRENT")
aprxMap = aprx.activeMap
layer = aprxMap.listLayers()[0]
symblayer = r"C:Tempreferencelayer.lyrx"
arcpy.ApplySymbologyFromLayer_management(layer, symblayer)

5. At this point the layer colour changed to be the very bright one obtained from the layer file.
6. I then saved the project using ArcPy

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aprx.save()

7. I closed ArcGIS Pro and then re-opened the project.
8. As expected the layer had the very bright colour.

I think the comment from @KHibma was probably correct in that the main difference between my code and yours is the attention I paid to using a variable named layer in two places rather than setting layer and then trying to retrieve a value from layers.