In the Virtual Network section of the course you mentioned "You can transfer data between subscriptions, deployment models, and Azure regions. This is used a lot to allow infrastructure scenarios that span multiple virtual networks but belong to the same company." Can you elaborate on what "but belong to the same company" means?
IMHO, it’s just a way of saying "a company can have its infrastructure spanned on multiple VNets". It does not mean you can’t peer two VNets from two different companies.
When you want to peer many Virtual Networks together, the first thing you must ensure is that none of address spaces of any of those VNets overlaps one another, otherwise it will create lots of trouble when traffics flow between Subnets of VNets that have overlapped address schemas.
Within a company, when they plan to build their infra with multiple VNets (means there will be peering between VNets), address schemas planning is carefully planned (usually handled by experienced networking administrators/engineers), and it is very unlikely that they will end up with address overlapping. However, that cannot be guaranteed if you peer two VNets of the two different companies, because it will always possible that their address schemas (or parts of them) may overlap each other.