First, you're probably correct. The BBEdit User Manual could use an update for something on instaproject so perhaps you could submit such a request to BBEdit support.
That said ...
If you search Google with "what is a bbedit instaproject," you'll get a pretty good AI Overview explaining what an instaproject is and how one can be created.
Also, searching this forum for variations of instaproject, insta-project, insta project, etc., you can find a concise description in the
Insta Projects thread by Jim Correia:
An Insta Project is just a project that doesn’t prompt you to save (unless you make modifications) based on how it was created.
You can create them by dragging folders onto BBEdit’s Dock/Finder icon, asking BBEdit to open a folder with a script. or by choosing a folder off the recent menu.
A slight addition/expansion to Jim's creation list is using the command line tool bbedit with:
bbedit path-to-folder
in the terminal or a shell script.