etymology - How did 'pummel' evolve from the meaning of apple?

The Etymonline entry that you cite is for pommel, the noun meaning "the ball at the end of a sword". Pummel, the verb, appears to mean to strike with, or as with, a pommel.

Johnson's A dictionary of the English Language of 1768 has an entry for pummel, but only as a variant of pommel (as a noun). It defines pommel both as a noun, and as the verb we now know as pummel:

To POMMEL: To beat black and blue; to bruise; to punch.

(Entry)

And Johnson and Walker's Dictionary of the English Language, published in 1828, has the following:

To POMMEL: To beat with anything thick or bulky; to bruise; to punch.

(Entry; emphasis added)