But on the main topic:
Yeah, we're not seeing a strategic secession to better fight Aurvandil. We're seeing the implosion of a realm. Hireshmont has abandoned all positions in disgrace, the Chateau is being TO'd, the seceding duchy just recently got pillaged and burnt, and the southern duchy may secede too, leaving a ruined shell.
NONE of this is strategic. It's every man for himself now in Terran.