This is false for Maine. It is a valid way to vote for a single candidate and not rank the rest. The ballot advises you not to do this, but it doesn't explicitly invalidate your ballot. See slide #6:

https://www.maine.gov/sos/cec/elec/upcoming/pdf/RCVMarkedBal...

You can vote for a single candidate across all your choices/rounds (across a single row), but voting for multiple candidates in the same choice will invalidate your ballot (down a single column). Multiples down a column would be like filling in all the circles for all candidates on a normal ballot, or even not filling any in. Same difference.