The player is the cause of the explosion, just like any of the player's skills. Anything killed by the explosion is killed by the player. Monsters killed that way have always triggered your on-kill effects, because that has always been considered you killing them.
I am literally telling you that the change made here was only to some descriptive text. It is not possible for that to change the behaviour.
There are two different kinds of on-death explosions in PoE. One type (which includes this mod) occurs regardless of how the enemy was killed, but requires that you be the one to kill it, as opposed to a minion or totem. The other type are things like Quecholli, which only explode things killed by hits (sometimes specific types of hits), but those hits might not come from you - totems can use your skills and thus deal hits that benefit from these stats.
Descriptions on these explosions have changed to make it clearer which kind they are - this mod and others like it have changed to specify that the enemies have to be killed by you, which has always been true for those stats. The other type have changed to specify they only apply to things killed by hits.
This is not a mechanical change. This always only applied to things killed by you, but now specifies that.