Inconsistencies with Deathfire Touch

I hope this is the appropriate board to post this, I don't use the boards much...

Deathfire Touch says in its description that it makes your DAMAGING ABILITIES deal magic damage over 4 seconds, then lists off the ratios and such, blah blah, that's not what this post is about.

Let's look at Lux for a moment. Her passive proc triggers Deathfire's damage. It isn't its OWN spell, but it's a damaging ability, even though it's her passive. Makes sense that it works on her passive, pretty straightforward.

Let's now look at Talon. His passive is ALSO a "damage on hit" ability, but does NOT trigger Deathfire, as far as I've been able to google/test for myself.

Not arguing in favor of the use of Deathfire on either champ, I'm not trying to make an optimization discussion, I wanna know if anyone on here knows why one Champion's ability (Lux) that works identically to activate as another's (Talon, in both cases using an AA after an ability or sequence of 3 ability hits) activates Deathfire Touch, but does not do so for the other Champion.

Any help, clarification, or what have you, is much appreciated.