As per the article, ISACTUALVERSION returns TRUE for Actual Version and FALSE for all other Versions.
However, I observed that the actual output is different when you use Time as a Dimension.
With Time as one of the Dimensions, the output is a function of Switchover month for each version – TRUE for periods before the switchover period and FALSE for rest, for EVERY Version, not just Actual Version.
This isn't unique to the ISACTUALVERSION() function. It's just how a line item is calculated when the "switchover" flag is enabled.
Time Periods prior to the Switchover Period in a version use the value from the "Actuals" version (which, for ISACTUALVERSION(), is always TRUE). It's also why the cell count for the line item increases/decreases when you toggle the switchover flag on/off - the cell doesn't actually "exist" for that version (it just takes the value from Actuals).