I'm always adding to the older than 120 days threads, I keep finding old recipes I want to try, lol.
yes that is where to add to them and then they come up in the new replies list etc but all old info is also there.
It is a basic bulletin board feature to have that warning (same as the so many people have replied since you first did message) meant for busy chat type sites.
But as a recipe or even chat on here is rarely likely to go out of popularity then it's not an issue.
Yes I believe it's not on to post recipe book recipes.
Not sure why you didn't get a new reply on that tweak, but I've found the new posts can be quite flaky at times so probably just one of those things. (smf can be quite flaky with that feature in general, though usually it's an emptying of new posts so you only get a couple rather than the load you haven't read).