In Gary Bernhardt's talk about Ruby and JavaScript surprises, what does "wat" mean?

Unforgettableid is correct, it is synonymous with "what" as in exclaiming surprise or bewilderment at a given situation.

I disagree somewhat with the quoted material, however, which states that 'wat' arrived in its current shortened form just for ease of typing.

Chat slang is a method of typing long words and phrases as short one-to-four letter words and is also used by people who have difficulties spelling. For example, instead of typing out "are", someone may only type "r." Although chat slang can be easier for you and sometimes faster to type, it makes it difficult to read and most people will ignore you.

I think the 'h' gets dropped precisely because it connotes a more sound-like visceral reaction.

I believe that the person exclaiming 'wat' is such a state of surprise that they are participating more in sound than language and want to directly connote this feeling to the reader in a kind of playful tongue-and-cheek way. My answer might be too in the realm of speculation or linguistics but maybe not, I'll give it a go.

After five minutes of online research, I found out the answer.

"Wat" isn't a sound: it's a word. It's a piece of Internet slang meaning "what"; in this situation, it's used to express disbelief at these particular idiosyncrasies of Ruby and JavaScript.

Of course, it's generally a silly mistake to use Internet slang words such as "wat", because it will confuse Internet users such as me. The Computer Hope website notes:

Chat slang is a method of typing long words and phrases as short one-to-four letter words and is also used by people who have difficulties spelling. For example, instead of typing out "are", someone may only type "r." Although chat slang can be easier for you and sometimes faster to type, it makes it difficult to read and most people will ignore you.

(P.S. The mainstream definition of the word wat — "a monastery-temple in Thailand, Cambodia or Laos" — doesn't apply here.)