Yes, I think the point is that the only reliable means of escaping Hel's grasp is by dying honorably in battle. In fact, it seems to me that it is so reliable, that probably a lot of lousy examples of dwarfdom, lives that well-deserve Hel's eternal attentions, escape their just fate by a small helping of courage and well-timed bloodshed.
In essence, isn't that Hel's gripe? That many many souls that should have been hers are going elsewhere based on a "technicality"? We have no particular reason to doubt that Hel is being quite truthful about how unfairly it all has worked out.
That said, IMHO, The Giant's comments have implied that others could "die with honor" by virtue of living genuinely virtuous lives in an honorable fashion. Of course, such is probably pretty hard, not something the average dwarf can necessarily achieve. Why count on that when you have a Sure Thing?
BTW, The Bet suggests to me that no dwarf truly retires from the military. Sigdi may be on pension, which means she has retired from the daily grind that an active member is responsible for. But, all dwarves have some expectation that a battle can be found soonish when their health beginning to fail. Not that all dwarves necessarily need such an out (see above), but dwarven society would be arranged such that the option was available to all.