I don't know what kind of "cache" plugins these software companies have, but I'm sure it's more for their public relations, not a real perspective. The real thing is behind a physical server only.
What you suggest would work only for guests anyway, and only if you don't have a lot of dynamic stuff on forums. But All PHP scripts are dynamic. miniBB itself is a very dynamic script. And if we talk about vBulletin... they even parse all BB-codes on-the-fly. Of course, some way they would need the script which "caches" themselves. But miniBB is very well cached itself in that case. Everything we could store static way, is already stored.
Some of the elements can't be stored at all.
Let's take Who's online module - it changes all the time, you can't store it in cache. That means, any page it is displayed on, can't be contently cached.
When you are logged in, on forum there appear different links ("Edit", "Move"), some new options (reply/new topic forms) and so on. They all depends on WHO is logged in - administrator, or moderator, or a regular user, or if nobody is logged in at all. If you use Signatures add-on, and if the user changes a Signature, that means, all older cached won't work anymore.
I know very well how Caching works and what it means from the perspective of PHP scripts. So... once again, we don't need this for miniBB. What we can optimize, is already optimized. If your Pentium IV server on CentOS works slowly with 400 visitors per second, not a wonder. Upgrade the hardware of the server. Upgrade the software on the server. Optimize the server itself. Purchase dedicated server. Rent a Cloud at least. Cache won't help you in that case.