What does single-tenant SaaS even mean? All of your company's data will be in a separate AWS account? Isolated Kuberentes cluster? Individual RDS instance? And how are you ensuring that the cloud provider you rely on is also single tenant? It is a meaningless term unless the service describes exactly how they are isolating customers (And it doesn't seem like they are in this post). In my experience the term is just used by salespeople to assure paranoid CIOs and nothing else.
I'll attempt to explain to clarify my own understanding.
SaaS is a delivery mechanism. Tenancy is an isolation model. To your broader point, the isolation model is implementation specific.
Multi-tenant SaaS means a single deployment for all tenants, and the data is delivered over the internet.
Single-tenant SaaS means a separate deployment for each tenant. I think common usage means a separate database per tenant. Single-tenant can also include entirely new infra for each tenant with private networking which is what Gitlab describes.
The press release says customers can pick the cloud and region they want to use so I suspect the answer will be 'yes' for those questions.
A ~1000 person company I'm familiar with moved from GitHub to a self hosted GitHub because it was too easy for engineers to hit a button and accidentally publish a repository/gist to the public. That shouldn't matter, but sometimes it does. I'm not sure if that's the same on GitLab.
Usually means each customer gets their own DB and instance of the app. This is a pretty good protection against a software bug from reading another customers data.
