If you believe "spinning up" a new box is required then it sounds like you don't know how containers actually work.
There are a couple of methods to make an application multi-tenant. One method is to spin up container instances that service a specific tenant (customer environment). The other is that the application is natively capable of operating in multi-tenant.
I would imagine that the former is being used - only - it may not be containers. It could be you're deploying directly from the CI/CD pipeline.
Nevertheless, it would make sense to use containers moving forward. Invision is still providing "self-hosted" software so while it does then it should invest in modern deployment technology especially where this makes sense to use internally as well.
I don't see containersing as adding no value - while no information is provided on how Invision runs it's cloud there aren't that many configuration on how it could be run efficiently and at scale.
Further, the cloud plans offered are quite expensive for the value provided. If this was different then yes you'll probably find most if not all customers would be on the cloud plans (except for one caveate, currently there are only two locations offered for hosting which may not be suitable for some customers).
Self-hosting will most likely be around for a long time unless Invision progresses the cloud plans quite substantially to make it worth the jump. The updates to the cloud plans have been extremely slow to say the least (I check on them from time to time).