No. There have been others to throw a nine-dart game finish.
The Daily Star refers to four others:
Borland becomes just the fifth player in PDC history to hit the perfect leg in a last-leg decider, joining Ian White, Chris Dobey, Mensur Suljovic, Madars Razma and Gabriel Clemens.
However the article does not elaborate on when exactly they did so. Checking each individual's history of nine-darters is non-trivial in some cases, but can be done.
The easiest nine-darter to confirm was by Chris Dobey. He threw the perfect leg against Dave Prins on 10 February, 2019, during the Last 64 round at Players Championship 2019 PC02 Wigan. This is perhaps more notable as it is his only nine-darter so far, and was also the eleventh leg of the eleven possible (therefore was the last possible leg in the match, not just the last leg).