I know little about geographical distribution, but rejecting
"The typical student in the program takes about six years to complete their course work,"
isn't the same as rejecting "singular their". "Singular they" referring back to complete noun-phrases is less widely accepted than "singular they" with indefinite pronouns. I predict that a sentence like:
"Anyone can bring their pets,"
would score better with the AHD panel (though I have no proof).
Similarly, almost none of the experts I know would accept singular their in the original post's context. "A dark figure" is a concrete reference, rather than "typical" reference. It's a definite reference (albeit a mysterious one).
"Singular they" is *not* about disguising gender, although this has always played a big role and has probably lead to the usage in the first place. You can find examples where there's no gender ambiguity:
"I know a mother will always love their children, but to the point a grown child may cause her to lose a responsible and loving husband and father that is crazy." (
here)
"I would think that a mother would love their children equally and unconditionally." (
here)
"The usual definition of a mother is often just a woman who takes care of their child. ... Although being a mother to their child is a social norm of our culture, Edwards defends the position that it is not morally or ethically wrong to choose an alternative over mothering because a mother knows best, even if it means giving up their own child. ... [and plenty of similar examples]" (
here)
"I never realized how much a mother could love their children until I had him." (
here)
What you need for "singular their" is not gender ambiguity, but an indefinite referent. So back to the original post:
"The black figure stilled for a moment, assessing Samantha, seemingly confused by her interruption, before they began to move again."
I doubt Phael is unique in looking for someone else, in this case. That's because unlike most other instances of "singular their" this one has a definite reference, albeit a mysterious one. Flat out wrong? I'm not sure. I've seen examples from native speakers (the only one I can think of now was in an unpublished script I'm not allowed to share), but I'm not sure how widespread this is. I'm thinking that this is a singular-they innovation, based on the assumption that "singular they" is about gender ambiguity (which does play an important part in usage history, but is neither necessary nor sufficient to use it).
"Singular they" is even more awkward in the above sentence because there are actually two people in the sentence who could plausibly both resume movement. Something like the following might be marginally better:
"The intruder made sure their footsteps could not be heard."
But there's always a hint of "more-than-one-person". I doubt you can get rid of that with a definite referent. And would anyone really say:
"Is 'Chris' a boy or a girl? I wish they had a different name."
Just bear in mind that not all singular theys are equally acceptable. I personally would never use it in the original post's context on account of its definite referent.