There is a difference between hyping another character or yourself and explaining your abilities. Unless the character has a mental deficiency, they know their own abilities, what they've done to obtain said abilities, and can explain their abilities to the viewers. It's clearly stated she ate all of Tartarus souls. You saying Mashima never showed anything regarding that is meaningless cause the only time the form was used was once for a split second until this arc lmao. Clearly in a prolonged battle, it's not surprising she is able to use their abilities when she stated to have absorbed all their souls. This faux outrage for something that was explained since the inception of the form doesn't make sense.
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.
Mashima doesn't like utilizing hax to the max potential. It's why DiMaria is a farmer now. Brandish comes and go for two seconds at a time. Irene had kill herself off like three different times. August suicide pack. Acnologia not utilizing time-manipulation after obtaining it. Why Mira doesn't go for Macro from the jump. Why Laxus doesn't spam Fairy Law. Why Ever ain't spamming stone eyes. 90% of the s12s in Alvarez could have won their battles if they just used their hax to a mediocre degree. He doesn't know how to effectively counter these hax so he doesn't even write the enemies utilizing it unless he has a way to effectively counter it. He even stated in an afterword that hax bust PLs and it's hard to write around them. The guy rather draw battles that involve physicality and explosion rather than intricate battles that really utilize a character's hax in an efficient way. So when he does, it's a surprise for sure and sometimes can be seen as bs, but it's not bcs he had explained said abilities beforehand. He just didn't utilize efficiently until now.
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.