It turns out that the code in the other thread you linked to is for the datepicker that appears on the page where you create events and set dates for sale start dates, event dates, and so on.
You can set a default date format to the registration form datepicker input by adding something like this to your site:
The support post ‘Date Picker – Change Format’ is closed to new replies.
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Support forum for Event Espresso 3 and Event Espresso 4.