Having given a concrete statement of “… always use the block form of if.” the PBP then gives you a time and place to use a postix if. It says that’s okay if you’re it for using a flow control statement like next, return, last, etc.
The goal of this exception is to make those control structures more visible, and keep them from being hidden deep in code. Fine.
I see this as similar to the exception I like at the start of the function, and find it a fine use of a postfix if.