1) Without a population distribution, you can't say how few.
2) It says they were taught. Not given.
3) Now we have moved from "there aren't enough who don't get it to matter". Funny how that just seems to keep shifting.
What, that a god can teach something that can eventually can become innate to the people?
Do you know what innate means? And it does not say that they were taught their innate magic. Nowhere. You're inventing a Strawman for the books now. Is there no line that you wouldn't cross to be right on the internet?
And so we get back to your imposing your logic on the world. You have decided that it is impossible to teach something that became innate to the race. That isn't something that we are told, and in fact that is something that is often done in mythology.
It has to do with teach vs. innate and what those two things mean. D&D 5e specifically uses common word usages and you are murdering those in your efforts to be right. You aren't. You are wrong. You are literally claiming that Baravar taught them the equivalent of a heartbeat.
You have also assumed that Baravar taught them wizard magic, and granted them additional magic above and beyond the magic they are known for. The book doesn't tell us that, that is your assumption. It just says "magic", which is what they have, and so what possible reason could there be to assume he taught them magic, and then gave them entirely separate magic?
Because arcane magic is the sort they are best at and it's the only sort of arcane magic(so far) that you can be taught. I suppose she could have taught them prayers for divine magic, but she certainly did not teach them innate magic, since innate magic can't be taught.
We know that not every Deep Gnome has this magic, both from using the NPC statblocks in the back of the MM and from PC builds. So, this innate magic can remain unavailable to the Gnome. Perhaps it is much like sorcery. Sorcerers can be born with innate magic due to living in high magic environments, but must be taught how to use that magic. Perhaps the Deep Gnomes were taught this magic, then simply used it so much and so often, it has become innate to them, but they still must be taught the proper ways to use it. This solution seems to work for everything, them being taught, it being innate, not everyone having it... except then their God taught them. Like I said and like the book said. Which would make me right.
Sorcerers are not taught to use their magic. At all. Period. They gain an innate understanding of how to use it. Again, your lack of understanding of what innate means seems to be working against you here.
"One can't study sorcery as one learns a language, any more than one can learn to live a legendary life. No one chooses sorcery; the power chooses the sorcerer."
Interrestingly, one of the ways to explain where Sorcery comes from is the
gift of a god. Not teaching, but gift. You can't teach innate ability.
You know, I can think of a great example too, after looking at the Forest Gnome entry on Illuisions. It states: "Forest gnomes have innate magical ability, letting them create simple illusions. They practice the use of illusion magic from an early age." Know what this made me think of? Cats. Cats have an innate hunting ability, they are natural born hunters. And yet, in the wild, their parents teach them how to hunt.
And yet you deliberately cut out the rest of that passage that says that because of that practice, almost every village has a full blown Illusionist and apprentices, showing that the practice is with wizardry, not innate magic.
And yet you are more likely to find bent spoons, broken pottery, floral drapes and other mundane items than a single gold piece or gem in a goblin lair.
They suck at raiding. They're goblins. They also take over abandoned mines and such, and would have that sort of stuff to scavenge from the abandoned areas.