That "It depends" is much less of a "hem and haw" than it is an "honest answer." It really does depend very much on a number of things, including but not limited to the choice of forum software, the mods/add-ons/plugin installed, the options selected in the operation of the forum, and the type of traffic.
Yep! And I suspect they are very likely telling the truth in most instances. 10-15 users an hour with a poorly scripted, inefficient, forum with "all the bells and whistles (even poorly programmed ones) enabled, huge avatars, the ability to attach images and files, etc. can bring a shared host to it's knees. Other software, well-programmed, light-weight graphics, limited (if any) ability to attach files to posts, etc. may be an entirely different matter. There is a "massive* difference in resource usage among forums ... not only that directly attributable to the software used, but also attributable to the way the software is configured and the patterns of the forums users.
Why would you think that? It seems to me that you are almost complaining that a webhosting company is being too honest with you by insisting upon qualifying their answer. I think they are doing the right thing in doing that, particularly given the lack of technical understanding and sophistication of many shared hosting users, in that they are being careful not to make misleading representations.
Just as soon as they make a "ball park guestimate" and say "We think you should be able to run N users", some twit will get bent because his crappy code can't run that many users on a shared host forum.
The same danger applies when evaluating others' claims regarding their sites - the operative rule here really is "YMMV".
This is a perfect situation for DreamHost PS, as you can dynamically "power" your site by tweaking your resources as necessary depending upon how your forum works. DreamHost even gives you some idea of what kinds of performance you can expect with a given resource configuration on their DreamHost PS page.