The parser of qemu-img actually skips several checks. It doesn't check for "# Disk DescriptorFile", it just skips over the first line with "#". It doesn't care what characters comes exactly after createType, CID, parentCID etc. it just assumes it's a =. It completely ignores "encoding" which could result in mishandling a completey correct file. There could be a lot more. This is just what I got at a glance.
The VMDK specifications I could find lack RFC level clarity. The slightly better documentations that can also be found are apparently written retroactively based on the code in QEMU. So I can see why someone would assume the quotes to be optional. It could be a simple mistake in the generating code though, like forgetting to escape the quotes so that they are actually emitted.
Example in shell code:
echo "some text" -> result: some text
echo '"some text"' -> result: "some text
I've opened a case with Veeam which creates such files. I can only image that VMWare handles the lack of the quotes gracefully. Either hat or nobody is using that. I've also submitted a patch to the QEMU developers, so that the value for createType is also accepted without quotes around it.