it is definitely an incremental extract. According to the documentation (snippet pasted below), table.incrementValue should contain the largest value in the current extract. However, in my case, it contains the smallest value.
"If this is an incremental refresh and the table supports incremental refreshes, this property will contain the largest value from the incrementColumn in the current extract. This value will be an empty string otherwise. See Incremental Refresh for details."
I faced the same issue with table.incrementValue not holding the largest value from the incrementColumnId field.
Fortunately, this was limited to the Simulator. When I imported the WDC to Tableau Desktop, the Incremental Refresh functionality worked as expected.
In Tableau Desktop from the Data pane on the left of the window, right-click your data source > Extract > Refresh (Incremental).
Hope that helps!