Good essay by Bret Victor, but he’s got one thing wrong in the opening chapter

I read Bret Victor’s latest essay, it’s very good. But he started off by refuting a quote of Alan Perlis which goes like this: “To understand a program, you must become both the machine and the program.”, and Bret thinks this is a mistake. I think Bret got that argument wrong, he is misunderstanding the issue that Perlis is addressing.