Technically true, but Nixon was merely hours from being impeached. He resigned before the full House took a vote to impeach. The House Judiciary Committee had already approved articles of impeachment. The Republicans basically acquiesced because they knew Ford was in place to succeed.
It is interesting the Democrats didn't follow the same logic in 1997 and let Gore become President. Gore would have had loads of incumbent President advantages for the 2000 election. Speaks to the raw power of the Clintons.