They may be listed in the alphabetical index but unless you actually tell us that, a newbie won’t know that. Been here a year and I didn’t know that!
I was also thinking about those who are working through the Lab module by module and started with the oldest one. Maybe they just finished Lab 6-3 and want to do Lab 6-4. That’s a lot of scrolling. With a jump link to the Lab 6 modules they won’t have to scroll as much.
When I designed web pages, it was all about how to make it easier for the visitors to find information and constant scrolling was always a complaint about many websites when I asked for input on design.
I still like what you are doing with it though, I just think it could be more user friendly.