Yes, it's correct. There is reasonably full explanation in the Eraser manual.
The reason is that Eraser 5 (which uses the same algorithms as but different code from Eraser 6) could corrupt the file system quite badly when erasing files compressed or encrypted by the OS. The Eraser team therefore decided, when implementing Eraser 6, to remove the capability to erase these files, at least until such time as the main Eraser design was fully implemented (which will not happen until Eraser 6.2 is released), and there was time to work out a reliable means of doing the job.
If there is enough physical space on the drive, compression can be switched off, and I believe that all files will then be decompressed. With today's large hard disks, the cost-benefit of compressed partitions is perhaps more questionable than it once was.