For all who look at the data in the URL, I agree that there is no explicit statement that one cannot have a 0 increment. I suppose the following line,

"If an identity column exists for a table with frequent deletions, gaps can occur between identity values. If this is a concern, do not use the IDENTITY property. "

infers that issue, if you add to the sentence "because the identity property may try to make a duplicate after you fill in a gap without proper coding to look for duplicates" etc.

If one does try to run the command, the actual error message that I got was "Identity column 'mainkey' contains invalid INCREMENT." So it's an "undocumented feature."

And no, I honestly didn't execute the command before I answered the question.