The theme of the engineering profession is to build tech-based items for sale and profit. The theme of a Tinker class would be to use tech-based abilities to accomplish class roles (tanking/DPS/Heal) or completing solo content.
Even thematically there is no overlap. It would be like comparing an Enchanter and a Mage. Both use magic, but they use magic for very different purposes.
If the purpose of the demonology profession resembled the engineering profession and it created robes and bred demonic pets for Warlocks to use in pet battles, then yeah it wouldn't conflict with Warlocks at all.You are just being dishonest and intentionally obtuse because of your little pet issue here. This is just as silly as saying "Let's add a Demonology profession and it won't conflict with warlocks at all!"
We should also mention that professions don't conflict with classes because a player who roll both a class and a profession on the same character.