Definitely agree that claims of being "open" are not just about supporting lots of cameras. But that's actually a big issue for IndigoVision. As a third party vendor with software that integrates with a range of other VMS, we found ourselves unable to develop a fully functional integration with IndigoVision Control Center due to the significant limitations in the partner SDKs compared with most other VMS. This may be changing over time, but the legacy IndigoVision approach (perhaps understandably, given the heritage of building locked down, closed end-to-end systems for UK police and others) of locking the video content and camera outputs is the antithesis of being open.
And then there's the existing customers with large numbers of deployed IndigoVision cameras from the 2000's who find themselves "trapped" with Control Center as the only VMS able to properly and reliably interface to their cameras (and thereby also trapped into buying more IndigoVision cameras in a proprietary lock-in).
The comparison with Avigilon is a good one (as Avigilon's Control Center is also the only VMS to be able to get the most out of Avigilon cameras in the end-end ecosystem model) but at least Avigilon's partner SDKs are genuinely open, really well documented, and easy to develop with.