This doesn't sound right. Does this mean that the drive decides to arbitrarily move blocks between your partition and unpartioned spaces to manage GC?
An SSD will still provide a virtual bitmap so file systems can still order it to write a bit at a given location like ye olde days, but the SSD controller ultimately decides where to actually write that bit which more than likely has no relation to the specified location.
Incidentally, yes: This also means defragmenting an SSD is not only harmful, it is outright misleading because the state of the file system is not related to the state of the bits on the drive.