VTL was an answer to tapes and it's linear nature.
Disk/JBOD or NAS was the next evolution to VTL. Removing the complexity of still managing tape, even if virtual.
DataDomain was NEVER a VTL only device. VTL was an option. Few folks actually used that option in my experience. More preferred ethernet. Only certain use cases dictated VTL whether it compatibility, comfort with tape, or comfort with the transport medium.
Symantec came along and really started to change things with OST. Which vendors like DataDomain, ExaGrid, and a few others supported. Remove VTL, use simpler/cheaper Ethernet, In some cases a highly optimized NFS protocol, in other cases an optimized packet/payload with other intelligence like optimized duplication/replication. This not only sped up backups, but allowed one to remove one more administration console by letting NBU manage replication and not the appliances.
The thumper back then was novel for what it was, a big NAS and ZFS was thought of the next big thing. It's shine quickly turned to rust with the acquisition of Sun by Oracle.
That being said, VTL is alive and kicking today. Some dedupe backup appliances still only support that. Some applications like TSM seem to work better with it. Some OS's can only use that (AS400/iSeries,Mainframe). Then there are some old school grumpy Unix dudes that still like "managing," tape...
The ideal solution can do BOTH. Act as a VTL if needed, but also the more sought after CIFS/NFS connectivity. All concurrently.
The pie in the sky solution gets rid of backup servers altogether, or at least getting them out of band, and being able to backup directly to your storage target. Then you can make multiple copies of that data via snaps, replication, clones, etc.
I suggest you take a look at the TechDay videos of Cohesity on data protection on youtube. Those guys get grilled and stomped on about data protection. The rest of their use cases are great, but data protection, no thank you.
This article would have been super back in circa 2007 or so. It seems to be missing a lot of relevant information let alone real world examples.