Okay, after some more experimentation, I was able to get it to work, but your instructions are so misleading as to be worse than useless. I followed them exactly in a couple of big threads to no effect at all. I eventually determined that it has nothing to do with the absolute quantities of posts as you state, and everything to do with the ratio of how many posts are scrolled past vs. the total thread length. It’s actually easier to get it to appear in really short threads.
The guiding principle for when it appears seems to be whether there is enough room visually on the progress bar to have a gap between the handle and the button, regardless of how many posts the user has to scroll past to get back to their previous position. Observe:
“Helpfully” letting me jump past one whole post to get back to the previous position in a short thread:
My latest-read position in this thread is post #3008. I can scroll up anywhere from 0-891 posts without having the back button or any indication of my read position appear…
…but scroll-up #892 summons it, and I can scroll to toggle back and forth between these states at will.
It also doesn’t work if you’re anywhere near the start or end of the thread (again by percentage rather than absolute post numbers). So if you’ve read 6,234 of 7,450 posts, it won’t appear no matter how much you scroll up, and the only way to get back to your last-read position is to scroll and pray or close and reopen the thread. The back button is a nice feature, but there are huge blind spots where it would be helpful but isn’t offered, apparently because a slightly cleaner aesthetic takes precedence over usability.
Okay, I may have gotten a bit carried away testing this out, but this sort of form-over-function thinking bugs me on a philosophical level. Figure out what functionality you want to offer, then make it as pretty as possible, not the other way around.