Yes, that technique requires a new, writable custom field.
But that requirement is kind of overkill if you only want to show the Full Name on the form and split it into First and Last (i.e. you do not wish to save the Full Name in Marketo). You can quite straightforwardly add a dummy Full Name textbox to a form
like this. Then you don't have any more clutter in the Marketo db if you don't want it.
One more note: don't expect miracles or more than 95% accuracy from that splitting function (I reused the same function from the blog post in my demo). It's not very culturally sensitive. For example, it assigns both middle names and compound (multi-word but non-hyphenated) first names to the last name field. So Norma Jeane Mortenson would have the last name "Jeane Mortenson," Anna Mae Bullock would be filed under "Mae Bullock" -- these are obviously incorrect. Sometimes making guesses on the user's behalf will just end up insulting them.