What does the writer mean by the sentence in bold? (He is a native Kiwi English speaker.)
The door code is now 1111.
Commence many frustrating/embarrassing mistakes.
Yes, this is just a joke. Notice the use of "commence" — normally used only in formal contexts, in an informal setting. The grammar is very relaxed, in fact, it is arguably ungrammatical, as you can't "commence a mistake". But not a worry because it's just a joke.
It's a slightly cheeky way to note that the change will probably result in many frustrating/embarrassing mistakes - jokingly phrased as if the writer himself officially "initiates" the mistakes.