Having playing and watched it, I would say that is false advertising.
There are quests for everything, including ultra-endgame (avaritia, extended crafting, creative items). There's a lot of quests and by the time you do them all, there is nothing left to do but build to your heart's content - which is certainly possible long before you "finish" the pack. Given vanilla minecraft is largely about freeform building absent any barriers, I guess you could stretch this to be the "fun" you get after finishing quests. Most people who actually finish it aren't sitting around building creatively in their survival worlds, though - they're going say "I finished the pack!" and move on to something else.
PO3 is an expert pack (even on normal) with altered progression. On normal, this isn't significant (although magic and some endgame stuff requires progress in more than just a given mod alone), on titan and kappa it is rigid. Since the quests are guidance for changes in the recipes, they don't lock you out of content per se, but rather indicate what now depends on what.
No one plays expert packs to have their fun after finishing it. For them, the fun is planning, logistics and expansion to satisfy the esoteric gates within the pack, ideally without resorting to grind or manual microcrafting. The stereotypical "journey, not the destination."