A13 You are somewhat correct. In a game with only items specifically specified by the DMG, those items appear with no +1 or other bonuses like that. They still qualify as magical weapons for the purpose of overcoming resistance to non-magical attacks, but do not have a bonus to attack/damage like +1, +2, etc have, nor how some other magic items DO have such.
That said, it's completely within a DM's abilities to make such items that have other bonuses as well. The DMG has sections on creating magic items, and they often involve mixing effects of other items to make something more powerful. So a DM could certainly drop an "enhanced" one and it's still not-quite-homebrew. It's a grey area. I would still call it RAW, just not allowed in some settings (like Adventurer's League).