L!Marth's weapon

The best understanding we came up with last night is that it's kind of like Sieglinde+eff except instead of using others' buffs, he uses his own, so basically you're correct

Siglinde copies nearby allies' buffs as combat buffs. Exalted Falchion gives you as combat buffs your own buffs, effectivelly doubling them. And also dragonslayer.

If it weren't for the dragon slaying part, Sieglinde would be still better. If someone runs dull close (as rare as it sounds), it supposedly negates Exalted Falchion because it makes your buffs 0 for the combat, but it cannot negate Sieglinde's copies since they are not visible buffs and they aren't in Eirika.

So they work similarly, but the change on who has the buffs changes a lot how they work. Also, for that part that changes, Sieglinde is much better since you can run her in an emblem team and copy a hone + fortify and Marth can't do that.

I get the feeling they are trying really hard to make Defiant skills a bit more relevant. L!Marth and B!Ephraim both benefit greatly from them. That's not to say there aren't better A slot skills for either of them, just that those 2 (and Bladetomes) actually make them useful on a budget.