The only problem with that is that Tiamat basically confirms that the Ascians were active during the Allagans era. It was the Ascians that compelled Tiamat to summon Bahamut in retaliation to the Allag, and it was the Ascians that gave the Allag the means to imprison Bahamut.
That said, Elidibus and Lahabrea were both described as "of this world", so it is certainly possible that either of them, or indeed perhaps lesser unnamed Ascians are "of this world" and perhaps were kicking around during various eras, including the Allags, and they "ascended" to Ascianhood at some point.