No, he's right. It's possible with some AMD cards to get more hash by overclocking core but not memory.
This is especially true with modded bios, the tighter memory timings sometimes already give maximum speed at stock memory clock.
Its card dependent, which was the conclusion I came too. You cant say either way, it truly depends on the card. With the R9 380X its not true that increasing core clock increases hashrate, but it may very well be true for other cards depending on how they are built and/or configured. My initial thought was that the RAM clock was the primary driver for hashrate, but as you and others have indicated, this is not always the case.