The Genius note expressed some skepticism about the veracity of this claim. Certainly, we haven’t SEEN Hamlet continually at fencing practice. But that doesn’t mean he’s not getting some practice in.
I mean, he’s preparing to revenge his father’s death by killing Claudius. What other kinds of preparation for murder might there be?
And when he does get the opportunity, he hits Claudius with just one blow – so he’s got some muscle memory in this department.
If I had plans to kill someone, even if I didn’t plan to murder them with a rapier and dagger, I might find rapier and dagger practice useful – just to get me going.
The Hamlet Project 