Scribe
Legend
If that is the reason the playtest omits the Duergar when describing the Dwarf species,
then
the Sea Elf is a Non-Elf species. Mordenkainen also lists the Sea Elf as a separate species.
In other words, there is no such thing as an Elf that breathes water (except when casting a spell to do so).
That said. I am comfortable with the Duergar being a Non-Dwarf species. I am also comfortable with updating the Duergar to be the same thing as the playtest Dwarf and making the spells come from a Duergar culture background feat.
The game doesnt support your view.
These are all variations on a species. There is no single Dwarf, or Elf. There are various species that are related and modified by either magic, or other external influence, to gain distinct biological traits and attributes.
That is not to say that cultural impacts could not also contribute to the definition of these game defined containers which are called a Species (capital S) Character (capital C) option.
This is also not to say that you cannot house rule, or develop your own system to replace that which you dont like. I built a framework of 14 Species last night, based on the Origins UA Template. Its painfully easy, and very quick, because Wizards has already removed so much that actually made a Species (capital S Game Construct) unique.
Monsters of the Multiverse is fully compatible with the next update of the game, per Wizards.