Doing things in a hex editor is a pretty weird choice. Is there no disassembler/assembler you can use? Translating the op-codes by hand would drive me insane.
But he's not modifying the code, he's just changing one ASCII char for another. And if you take a look at the hex editor screenshot, you'll see that it's not even inside the code, but rather surrounded by the menu entry names.
I did use a disassembler, but I didn't mention it directly in the blog post - rather it was at the link to 68KMLA. Not really sure why I did that, but I guess the blog post was long enough already. So, I just edited the blog post to mention it and make that more clear. Cheers!