Reading the data sheet and your code example makes me think this statement is not true.

The FLASH as i have learned is not the same as EEProm in that you cant just change bytes values as you may like to on the fly , changing byte to $FF requires your do a complete sector erase , depending on the chip and the size the sector erase can range from 4k bytes to 64k bytes at a time.
and a write to any location is possible. But I will keep reading