Important! Play-testing for Heroes of Might and Magic V – we are looking for volunteers.

I think they have almost finished the game completely; they are just polishing the details now...

That's why they will release a special, cut-down version instead of the full Heroes game. It will be fully completed and debugged. It will have almost nothing in common with the main game, except for the graphics and possibly the interface.
They won't let beta testers play the full game. They're not fools.
This is a widely used practice. For example, all automakers use special versions of their cars for such tests. These versions share the body and interior with the mass-produced product, but the engine, chassis, and tires are slightly different. As a result, they create a good image before the start of sales.