Apparently, both Escalation and Stronghold Assault are not expansions in the way Planetstrike was. Then again, according to some rumours, Escalation is just mini-Apocalypse, apparently without any modifications to the rules for super-heavies, gargantuan creatures, strength D weapons etc. It just has some changes to victory points and initiative seizing. And Stronghold Assault features fortifications which instantly fold to these super-heavies, but are otherwise almost indestructable to "normal" forces.
Then again, these are still rumours.