Arthuria I never even used the word "taunt" and of corse it is not enough to hold aggro.
Every aggro skill of knight makes magical dmg so you can use these on magical boss 3 and boss 1/magical adds boss 1.
But knight also has skills that do physical dmg. I spam disarmament on physical boss 3 (that is enough with aggro buffs, use threaten on magical boss 3 to get the buff from it.
For the physical adds from boss 1 you can use the aoe knight itemsetskills, they do physical dmg.
If you only do whitehits you may not hold aggro at boss 3, but as tank you use your skills not only whitehits.
At boss 1 the aoe come in an intervall so if you are skilled you can move a lot and take part in boss fight. It is just rly easy with ranged classes.
About your videos:
Fist of all: we do not know what equipment each person had so we can't compare their dmg.
First Video:
They were lvl 90 in a lvl 87 instance, wow.
Second Video:
Hey ppl from my old server... Kythago will most likely have failed again and the scout makes way more dmg than the knight. Oh and lvl 95 in a lvl 90 instance, nice.
Third Video:
Most dps in that group start later then knight. Oh and wait, its a 18 men party for Taren and he dies after a few seconds. Yeah i see how you compare dmg in that situation.
Maybe knight/mage makes a bit more dmg than other knight combos but it is still mainly a tank, not a dps that is equal to other (real) dps classes. So just be happy that you can make a lot of dmg in hos and farm arcadia stats to sell.
Best regards
Gjiara