No, he means "nominative". In "he is one of us" and "er ist einer von uns", the verbs "is/are/am/be/being/been/was/were" and "sein/bin/bist/ist/sei/war/..." take a predicate nominative as their complement rather than an accusative or dative complement that most verbs take. That's why people often correct "it's *me/him/her/us/them" to "it's I/he/she/we/they." In this case, "einer" is a masculine nominative, like "der".
That's true for the indefinite article (before a noun), e.g. ein Mann.
But the pronoun (that stands alone, without a noun after it) is einer in the masculine nominative.
Another example: Tom ist ein Mann und Paul ist auch einer. "Tom is a man and Paul is one, too."
You are the best !!! And if you know the Bosnian or Serbo-Croat language then you are the best of the best
einem is dative case, but "to be" takes a predicate in the nominative case, not the dative case.
So it's er ist einer von uns with einer in the nominative case.
(Note that the pronoun einer - which stands alone - has an ending in the masculine nominative, unlike the indefinite article - which stands before a noun - which has no ending there, e.g. ein Mann.)
Because it stands alone here, without a noun after it -- it's acting as a pronoun, not as an indefinite article.
Compare, for example, Tom ist ein Mann und Paul ist auch einer. "Tom is a man and Paul is one, too". The first one is ein because it's in front of Mann; the second one is einer because it's standing alone.
Possessive adjectives and possessive pronouns have this difference, too, for example Hier ist mein Pferd und hier ist deins. "Here is my horse and here is yours." -- the second one is not dein as in dein Pferd (possessive adjective before a noun), but deins with a neuter nominative ending -s (possessive pronoun standing alone).
You'll note that English does this for possessives as well, distinguishing between "my pen and yours" or "your pen and mine", rather than "my pen and your" or "your pen and my" or "mine pen and yours".
Does anyone else picture an evil German scientist going "He ist von of us"...or is that just me?
Well, ein(er) here is clearly not serving its normal role as indefinite article. It has been "nounified", evidently now a pronoun. I guess?
So a she would be "sie ist eine von uns"?
eines or eins are both possible there.
Es ist eines seiner besten Werke / eins seiner besten Werke.
ONE OF US ONE OF US
The only dative element here is "uns", because it follows the dative preposition "von".
I'm happy with it being nominative as described above, but why is it 'einer' and not 'ein'. Is it somehow related to belonging, i.e. possessive pronouns?
No, not possessive in this context. It is nominative, because the sein-verb "ist" has no direct object.
Think of the words "ein" and "Einer" as two separate words. Use "ein" as the possessive "a/an" and use "Einer" as the noun "One".
Another angle on this example would be if it were negated. In English: He is NOT one of us. Now, in German you could simply negate Einer with "nicht" placed in front of it, but that sounds awkward. It is more grammatically correct, and sounds better to say: Er ist keiner von uns.
Tl;dr: "Ein" --> "a/an" "Einer" --> "one" (use it as a noun but don't capitalize)
It seems it could easily be translated "he is one from us" is that not true.
No, because "uns" is not possessive. I'm thinking it would have to be "Er ist einer von unserem" but my grasp of dative prepositions is tenuous so I could be wrong
When it’s used as a pronoun (not before a noun but replacing one), ein takes endings like dies, e.g. einer and ein(e)s for masculine and neuter nominative.
einer is not dative. It's masculine nominative singular.
It's not followed by a noun -- it's a pronoun standing on its own -- and so the form is different from that of the indefinite article.
For example: Das ist ein Mann und das ich auch einer. "That is a man and that is one, too."
The first one is ein because it's before Mann; the second one is einer because it stands by itself.