Yes - it is true. Is it recommended - well - as they say - that's an whole 'nother question.
Let me back a bit - RAID1 is not a backup, as my signature says, it's about disk redundancy, not backup - so even if you use RAID1, you still need to back your data up.
On to your 'other' question - personally, the only two disk configurations supported by the DNS-323, that I consider 'useable' are standard volumes and RAID1. The options are ...
Standard volumes - this gives you one volume per disk, no redundancy
JBOD (linear) - first, there are different definitions for JBOD and DLink, for what ever reason, has chosen to implement this configuration in what they call JBOD (linear) - you get a single volume which has the capacity of the two disks, the data is written to one disk and when that is filled, it will be written to the second - this is only useful if you have datasets that are larger than the capacity of a single disk (and if you do have databases exceeding the capacity of today's available drives, then I'd say you really should not have bought a consumer device. The danger with this configuration is that if a drive fails, you lose access to ALL the data, even if the data is stored entirely on the remaining drive - that's right - if you store 500MB of data in a 2TB JBOD linear array, and the second drive, which has no data, fails - you lose access to the data - if not immediately, at the next reboot. Think about it - is the added capacity worth the risk - personally I think NOT.
RAID
A RAID0 configuration supposedly allows for faster read/write access by eliminating the disk interface as the bottleneck - this works very well in situations where the disk interface IS the bottleneck, but on a NAS device, the bottleneck is, more often than not, either the network or the controller, and this device is no exception - a RAID0 configuration provides no performance gain, and when coupled with the fact that failure of either disk results in the immediate loss of ALL the data, personally I see no point in placing my data at unnecessary risk.
A RAID1 configuration mirrors all of the data on one disk to the other and allows continuous access to the data in the event of the failure of a single disk.
On this device, if you create a RAID array - either RAID1 or RAID0 - that does not utilise all of the available space, the unused space will be configured as a second volume - however - in the event of a disk failure, when you replace the failed disk, and the unit rebuilds the array (in the case of RAID1), it does not recreate the second volume, and the only way to fix this is to back up your data, delete & recreate the array and the JBOD and restore your data.
Now you have they why's & the wherefore's - you decide what's best for you - for me I'd say NO to JBOD and NO to RAID0