Does the attack IV stat matter on a defender Pokemon?

It depends. For instance, for Blissey/Chansey the attack IV are much more important because her base attack is too low, so the little gain on attack would impact much more than on defense or HP. To a lesser extent, this applies to Snorlax too.

For a attacking pokémon, less defense IV means that pokémon will receive a little more damage, and so, will gain a little more energy for your charged moves, but also means it will faint a little faster. So, in this case is basically a matter of bulkpoints.

As ever, the answer is 'it depends'. But truthfully speaking, it's a 'no' from me.

I'm not saying a 0/15/15 defender is same as a 15/15/15, but the whole point of attack IV is to determine breakpoints for an attacker going into a raid. With maximum optimal breakpoints/bulkpoints you may be able to pull off that Lugia Trio in Rain or get that 1 extra damage ball.

With defenders, this is almost completely irrelevant. There is no award for hitting the next breakpoint in damage to the attacker. Of course, for elegance, you can choose the appropriate IV threshold that suits your requirements.

I maxed a 14a/12d/11s Shadow Claw Gengar few weeks ago. Yesterday I found out my second Shadow Claw Gengar was 14a/4/15...longest time I thought it had 9a/10/15. However, at the same time I realized it is the old move that 100% made the difference, definitely not 14a vs 9a

It only matters in one case - when trying to take on raids with minimum crew and the margin is a couple of seconds.

Otherwise it has no relevance at all, be it 0% or 100%, you will not see a differences.

So long as that Pokémon deals damage attack matters be, so long as that Pokémon takes damage defense matters. Not anything significant though. Most of the time difference is nullified due to rounding damage.

Assuming your attacker has low defense, your Pokémon ends up losing more hp.