This is intentional, even if unexpected, as shown in line 152 of the link (turn on line numbers in the settings menu). I agree that following PEMDAS would make more sense.
Edit: it's the equation below "Although if you want to find roots higher than 3..."
Maybe the author did intentionally break order of operations for shorthand on other roots, maybe they just forgot that's not how it's normally parsed. 152 doesn't comment on intent one way or the other, it just demonstrates how it currently is.
Edit: it's the equation below "Although if you want to find roots higher than 3..."