Perhaps the numbers listed are based on the assumption that players wouldn't bother with proper team composition and just try to brute force their way through events?
Elemental weakness/advantage on its own is you take about -25% damage if you're resistant and about +45% damage if you're weak.
Interesting thing is that if you look at the provisional damage formula, weak point correction is lumped together additively with the other element related bonuses. Things like Apocalypse/Phoenix/Garuda's bonuses to specific elements, Ignited Dragoon's summon effect of +fire attack/+wind resist, Agni's +fire attack buff. So the more you can stack up +element attack stuff, the less relatively impactful elemental advantage becomes for you as an attacker.
On the other hand, if you stack up +character of X element stuff (which stacks additively with assault instead) instead of +element, elemental advantage retains the same relative impact.
On the defensive side of things though, since specific +resists aren't as easily stacked up, avoiding weakness is still the name of the game since eating an extra ~45% damage for free is never fun.
Any character effect is multiplied by elemental effects. Character effects include the following: active and passive bonuses from certain eidolons, assault skill, attack up status effects, and assist skill passives. Meanwhile, elemental bonuses are more difficult to come by. To maximize the product, you optimally want the sum of all character bonuses and elemental bonues to be equal. With a full weapon grid of 8-10 SRs with assault skill, you'll have a minimum of 28% character bonus at LV1, 44% at LV5, and 64% at LV10. Character bonuses from primary and friend eidolons are only optimal at low assault skill (<20%) vs. weak element, and even then you'll probably want to pair one character-boosting eidolon with one element-boosting eidolon. Character bonuses from eidolons become undesirable at 80% assault skill or higher. Subtract 40% from that if you don't have the strong element bonus.
Is Typhon a fire boost only? Or is it dual element like the last three (dark/fire, light/thunder etc).
Fire only
From Typhon onward, almost every event eidolon will be single element. There will be two exceptions way off in the future, and one of those two exceptions doesn't raise attack. The other exception is part of a series that has its own gimmick.