Who or what does the lightning damage from a Javelin of Lightning?

This question asks about ways for a Tempest cleric to use their Thunderous Strike class feature.

Thunderous strike says (emphasis mine):

Thunderous Strike. At 6th level, when you deal lightning damage to a Large or smaller creature, you can also push it up to 10 feet away from you.

One answer suggests various magic items, including a wand of lightning bolts and javelin of lightning.

The wand grants you the ability to cast a spell, and your spell means it is you doing the damage.

It is not as clear to me that this is the case with the javelin, however (emphasis mine):

When you hurl it and speak its Command Word, it transforms into a bolt of lightning, forming a line 5 feet wide that extends out from you to a target within 120 feet. Each creature in the line excluding you and the target must make a DC 13 Dexterity saving throw, taking 4d6 lightning damage on a failed save, and half as much damage on a successful one. The lightning bolt turns back into a javelin when it reaches the target. Make a ranged weapon attack against the target. On a hit, the target takes damage from the javelin plus 4d6 lightning damage.

By standard use of "you do damage", the weapon damage from the javelin itself is done by the person who throws it. But if the javelin is transforming itself to cause the lightning damage, is that damage still done by "you" (the thrower) in a way that would trigger Thunderous Strike, or is it rather done by the javelin, and independent of the thrower?

Related: What is damage you do vs. damage a creature takes?