Due to the rules change on planeswalker damage redirection that thread is now out of date and incorrect.You cannot redirect the damage from Atarka's Command to a planeswalker.
Note that Atarka's Command's second mode didn't change with the Dominaria update (it still says "each opponent", not, say, "each opponent and each planeswalker your opponents control"). In general, the Dominaria update didn't change any "each opponent" or "each player" in the text of spells or abilities (examples include Atarka's Command, Earthquake, and Flame Rift; review C.R. 108.1; see also this thread). Moreover, with Dominaria, the planeswalker redirection effect in the rules was eliminated (C.R. 306.7 now says that this used to exist previously).
Atarka's command and planeswalker