Well you know that I was joking right?
But anyway, statements from characters are not always true or make sense, even if they talking about themself. What Franmalth said was his own ability after he absorbed Hades and what he said was not true and also doesn't make sense.
It's not clearly stated, many demons were already dead, Alegria is not a fusion of all demons, it was a demon itself. It doesn't make sense that she can use all curses with that form. Also, what is the reason that a Seilah form exists in the first place? If she can use all curses with Alegria because "she ate Tartaros" then makes Seilah form not much sense, same with Alegria, because Alegria was never a fusion of all demons.
Well imo it was more because he had no idea how to use their abilities or because of his own laziness or that of the publisher. There are enough possibilities how to use the power of the S12 and also how to defeat them. It' just that Mashima and other authors don't like it to use other characters than the main cast. What is a shame because this was one of FT (manga) strongest points, the balance between main and side characters.
It's true that many characters never showed us their fullpower or special abilities not only in FT but overall in shonens. Just like Erza doesn't use her Nakagami armor against Irene, Laxus or Misaki.
But that still doesn't change the fact that it doesn't make sense that she can use all curses with the Alegria form. Like I said, other forms like Seilah form would not make sense anymore.
You can like it if she can use the curses or not but this doesn't mean it makes also sense. Imo it doesn't make sense it all even though I like it that she can use the curses. But it would make more sense if she would use at least the forms of the 9DG like the Seilah form.