The problem is even _root_ can't add the missing /usr/local, because SIP prevents even root from touching /usr. SIP doesn't block /opt; if it did, MacPorts would also have broken, sudo or no sudo.
SIP doesn't block /opt for the exact reason MacPorts relies on /opt: It's not part of the existing operating system. In other words, this situation isn't just good luck for MacPorts, it's a direct result of the design decisions behind MacPorts, and the design decisions behind homebrew.
The specifics of System Integrity Protection are that no user, application, or process will be able to write files or modify files in the root System folder or the /bin, /sbin, and /usr directories, which are hidden by default in OS X’s Finder. The /usr/local folder remains accessible, however; it’s a long-running convention in Unix and variants as a place to stash material and software that individual users rely on.
The bit you quoted doesn't contradict what the OP stated, if /usr/local _does not_ exist, not even root can create it.
If it exists, we can use it as before.
Considering how many times I've installed OS X, I can't recall if /usr/local is there by default (excluding 10.11 for the moment). Will have to do another few installs and check.