The archer failing to do this and ignoring the aggro he was generating could be due to either of two different reasons: Either he simply wasn't very good and didn't understand the damage dealers' role in controlling aggro, so was going all-out too soon and to the wrong targets. Or he was over-leveled and over-geared enough that he simply didn't care whether he got aggro away from the tank (which leads to acting much the same as those who don't know any better).
In low-level dungeons, damage dealers may not want the attention of the final boss, but most of the trash mobs along the way they'd be able to take out just as they could while soloing open world mobs. Many get lazy about aggro control during runs they regard as easy, not caring about letting the tank establish hate first. And that's just because the aggro they take isn't likely to turn into enough incoming damage to really matter to them. It's not a good practice to treat dungeon trash mobs as soloable and ignore the basic tactics of role based group cooperation, but it's a rather common one, especially in low-level dungeons.
The problem with ignoring normal group dynamics during "easy" content is two-fold. For one, it establishes bad habits that have an unfortunate tendency to creep into other, harder runs where the mobs are much more dangerous. And for another, different players regard different content as easy or challenging. For instance, in the run the OP describes, the archer may well have regarded it as a simple low-level run, but to the gladiator, it was the first ever dungeon in the game and he was trying to learn how to tank properly. Whenever one player is ignoring group dynamics, it makes it much harder if not impossible for anyone else in the run to work on establishing proper group dynamics.
So I advocate treating every dungeon run, even the easy low-level Roulette ones, as an opportunity to practice your best group dynamics strategies. It reinforces good habits for yourself, helps to teach good habits to the other players you're with, (and in the meantime makes the run more interesting, as there are more things to pay attention to than when facerolling your way through it).