Music disc "Creator" and the heavy core do not have the same rarity. They should have the same rarity according to the guidelines from 24w33a:

• The chance of finding it in a loot table, including mob drops
• The amount of travelling required to obtain it
• The difficulty of any obstacle which must be overcome to obtain it
• The possible amount of that item or block which exists in the world

all of these factors are the same for both items, but they have different rarities (both items have a 7.5% chance of dropping from exclusively ominous vaults)