Taion explicitly states he was "taught" how to use the Mondo by Nimue, but elsewhere else in the game, the type and power of a Blade is spoken about as if it's some inbuilt quality of the people of the kingdoms, and being able to change past a certain point it is remarkable to the point of being unheard-of.
Based on this, I think Taion’s “Blade” is just the gauntlet he uses to manage the Mondo at a large scale, similar to how Noah’s blade becomes a gauntlet when he has Lucky Seven drawn. Nimue, similarly, just has the gauntlet, possibly due to having been a non-combatant in the beginning. Katashiro/Mondo being shared knowledge between them marks them as something else than a Blade, possibly a technology/combat skill that is lost to the rest of Agnus while persisting in Nimue’s colony through tradition.
Furthermore, it’s entirely plausible that Katashiro were originally not a weapon at all, but rather a surveillance tool. Taion being able to slap a bitch halfway across Aionios with his could be simply him being a talented mage -- something that is fairly rare among Agnians, whose combat abilities are more melee-focused with the occasional purely-ranged gunner thrown in. We also see him use the Mondo for non-combat purposes like tracking, when we know Blades will dissipate if knocked out of their wielder’s hand for long enough.
Agnian Blades also generally have a wider variety in construction than Kevesi Blades, possibly due to differences in Kevesi and Agnian physiology. Kevesi need power frames for circulating ether in their bodies, and for properly powering their Blades -- but with a limited amount of engineers in the world, the majority of soldiers won’t be able to customise theirs, or keep it tuned to the changes in their Blade. Agnians, in contrast, have core crystals that handle the circulation of ether naturally, leaving more freedom for naturally-occurring changes in Blades based on the wielder’s strength and temperament.
Perhaps due to a quirk of fate, Taion just happens to have both the powers and the temperament to make him basically the ideal Katashiro user, to the point that his original Blade basically just serves as an interface to allow him broader and more complex control over the totality of the Mondo he can summon, which is why the Ouroboros powers latch onto them. It puts him in an interesting halfway-category between the fully melee frontliners and the fully ranged rearguard, as is befitting of his primary role as a tactician and a support mage.