A few things. Twin Fangs IS treated as an "AoE or DoT" for the purposes of items like Rylies, Spellvamp, and Liandry's. However, that doesn't mean it won't benefit from those (with the exception of Furor enchant), it is simply a reduced effect. In all other ways it is considered a single-target spell.
That out of the way, the last, and most important thing, is that I think catraven is severely mis-representing ranges. First of all, Twin Fang is longer range than Apprehend, by 150 units of measurement. That distance is fairly small, being crossed by Darius in about 0.4 seconds ( a little less) with most boots by D. However, the cast animation for Twin Fang is shorter, so you can still kite, especially if he's slowed by Miasma or your Ult or your allies.
Her Ult has the SAME RANGE as her Q and W, so I have no idea where you get "melee range" out of it. Certainly, Q and W might look longer (since their range is for the center of their AoE) but calling 850 "melee" is disingenuous, or ignorance of the ability at best.
Finally, if you're really going 1v1 against Darius, don't use Twin Fang (or at least don't use it as you normally would). Take advantage of your superior range, slows, and speed buff from Q to maintain "Proper Distance" (on an unrelated note, scew Nine Breaker and it's "Proper Distance"). Nothing's going to force you to fight at close range if you are attentive (even if he Flash Apprehends you should be able to get away with your skills or your own Flash, if you can't live long enough for your slows, Ult, or Q to make a difference it isn't a problem with her kit, but rather that that particular player was too far ahead in items/level).
I don't think that Cass is particularly good, I'd say at this point that she's a little above average, but still not top tier. However, I very much dislike when people put out false information, or otherwise misrepresent how things actually work.