Untrue. NYC is heavily RAT but Republicans can and DO occasionally win office there, especially in places like Staten Island. Rudy Giuliani certainly was not a "token and unknown candidate" when he was elected Mayor of NYC mulitple times, nor was lifelong RAT Michael Bloomberg when he conned the NYC Republican Party into nominating him and was then elected as a "Republican".
Nicole Malliotakis, Mayor Warren Wilhelm's opponent in the last election, was not a "token or unknown" candidate either. She is a pretty prominent elected Republican in the NY State Assembly and got the job by defeating an RAT incumbent.
You want to see a REAL one party system, check out Chicago, where there hasn't been a GOP mayor since the 1920s, and the GOP hasn't even BOTHERED to field a candidate for Mayor since 1995.
Although Nicole Malliotakis lost citiwide in her mayoral run against DiBlasio, she crushed him within the lines of the NY-11 congressional district (Staten Island and parts of South Brooklyn, including Bay Ridge).
Malliotakis is running for Congress in the NY-11 this year, and has an excellent chance of regaining the seat for the GOP, once again (and as has been the case for al of four of the past forty years) giving the party control of a House seat located completely within NYC. (By contrast, the last time that the GOP held a congressional district lying completely within Chicago was in 1995-97 after Michael Flanagan upset crooked Democrat Dan Rostenkowski, and prior to that it had been a while.)
Curiously, while the NY-11 has the highest Italian population of any CD in America, its Representatives since 2009 have been German-Irish Democrat Michael McMahon from 2009-2011, German-Irish-Italian Republican Michael Grimm from 2011-2015, Irish-Polish Republican Dan Donovan from 2015-2019, and Jewish Democrat Max Rose since 2019; hopefully the streak will continue with Greek-Cuban Republican Nicole Malliotakis starting on January 3, 2021.