It is for our most avid players who might have sold the Fortress of Revenge beforehand. Due to this chapter's nature, the possibility of selling the Fortress of Revenge before completing the chapter is trivial but present. These players will still be able to complete the Questline. The game needs to check that the "Ultimate Power" is activated, hence the wording.
... Being still in mid-Halflings, I know virtually nothing about the mechanics of the game's final Chapters, including Ch.19 - but thinking about it, it's quite interesting that the Ch.19 Questline can be completed
both by the 'traditional' route (for most players?) of methodically completing Researches roughly in order
and the less-traditional (and I'll bet even more gruelling - I am at least aware of
that aspect of Ch.19!) route of selling the Guest Race Portal (which I'm assuming the Fortress is?) at some point before the Chapter is otherwise completed, and the next Chapter is opened up, so those players who haven't completed some, any, or all of the previous Chapter's Quests can simply cancel them.
After all - there IS no Chapter beyond Chapter 19, at the moment... although when there is, I'm assuming that Inno won't be including any Quest which requires selling the Portal for a reward - wow, that would be, er, annoying [politeness is the thing... haha]. In fact, and since I really do know SO little about Chapters 16-17+ ... are there
any 'Sell Portal' Quests in those later-game Chapters at all, or do they stop entirely by that point?
I myself am not planning to advance much beyond the vital 'All-3* Merc. Camp Troops' point in any hurry, if at all... well, unless as-yet unforeseen future game alteration(s) can make those terrifyingly vast Squad Sizes - and lengthy, Resource-demanding Researches -
very worthwhile indeed...!