First, look at the comic. It’s the second speech bubble.
First, a rule:
Who is some kind of a subject
Whom is some kind of an object.
Is the word a subject or an object? How else might you say that sentence?
- You could say, “Who are ‘they’?” That would make the word a subject, so “who,” not “whom.”
- You could say, “They are who(m)?” Since the verb is a form of “to be,” the word is a predicate nominative, so we still get “who” not “whom.”
- Maybe look for an antecedent, which would be in the speech bubble in the upper right. That has “they’re doing,” short for “they are doing.” Still a subject, so we’re still stuck with “who,” not “whom.”
The gal in the glasses is incorrect, using a pretentiousism. Maybe she takes after her mom, who also makes lots of mistakes.