Could Esri consider improving the Arcade developer documentation on joined tables?
How do I reference data from a joined table?
You can reference attribute data from a joined table using the square bracket syntax for referencing field names: $feature["joinKey.fieldName"].
// data from a joined table
var demVotes2016 = $feature["COUNTY_ID.VOTED_DEM_2016"];
var demVotes2012 = $feature["COUNTY_ID.VOTED_DEM_2012"];
// returns % change in votes from 2012 to 2016
Round( ( ( demVotes2016 - demVotes2012 ) / demVotes2012 ) * 100, 2);
That blurb seems misleading.
More info here: Arcade joined tables: joinKey
Thanks.