Can we all agree that the sentence in the title is an example of overcorrection? 
Depends on whether or not you know whom Sydney is.
Well, now, that depends. It may be undercorrection if the speaker routinely says “See that fellow over there? Him is Sidney.”
Especially if Sydney’s a girl.
But surely the writer was thinking “it needs to be ‘whom’ because I would say ‘Sydney is him,’ not 'Sydney is he.”
Sounds like the speaker went to night school.
What we have here is a case of the predicate nominal. Sixty years ago and earlier, when people were still insisting that English should sound like Latin, it would have had to be “Who Sydney is,” because English used phrases like “It is I!” and “Yes, this is she.” Today we use the objective case and say things like “Hey, it’s me,” and “Yes, this is her.”
The sentence in the title is tricky because it depends on which standard you follow, today’s or your grandmother’s. For some reason, even though most of the English world has moved onto using objective case in predicate nominals, it’s my impression that the old way still dominates in this situation.
Yes, yes we can.
I dunno. It could be an incomplete sentence.
“I don’t know whom Sydney is [talking about].”
I haven’t the slightest notion why any educated English speaker would think “I don’t know whom Sydney is” was remotely correct, notwithstanding the general acceptability of “It’s me” (shared in French, though they use the indirect object <<C’est moi>> and would no more think of saying <<C’est je>> than of eating American cheese); nor do I understand why talking about the predicate nominal offers the slightest excuse why “whom” would make an ounce of sense. Enlighten me!
Yes, it’s overcorrection . . . same as when people say “he and I,” regardless of context.
This. “who” is the masculine singular form; “whom” is the feminine singular form.
Thus, “Him, who am Sydney” or “Her, whom am Sydney”.
If multiple Syndies, then “Them, what are Sydnies.”
Yes, it’s over correction. Please contact myself if you need further clarification.
Sorry?
He went to the store.
I went to the store.
He and I went to the store.
The first two check out, so the third also checks out. Context kind of does matter.
Or do you mean it should just be “we”?
Or, in certain latitudes, “He’un and me’un went to the store.”
panache45 is referring to a sentence like “The committee assigned he and I to the task.”
Yeah, but that’s context. He said “regardless of context” which is where I got confused.
The point being idiots routinely use it in the wrong context. You provided an example of the right context, which is unremarkable, but lots of folks will blithely say “they listened to he and I” even though they would never say “they listened to he” or “they listened to I.” They are overcorrecting after being chastised for saying “him and me went to the store” – meaning they heard the criticism for their mistake, but didn’t understand it.