No, that is not a safe assumption, at least from the module name to namespace prefix mapping point of view.
When packages are split up, the old namespace prefixes can be kept for backward compatibility.
Magento recently approved a design document that states that will be used a lot:
In that document, the accepted section „Module naming“, paragraph 2.iii states:
Preserve old namespaces. Classes in CatalogAdminUi, CatalogStorefrontUi, CatalogCron, etc will have Catalog namespace ✅.
▪ No overhead from preserving keeping backwards compatibility.
▪ We going to have namespaces that don't follow the pattern.
▪ Will need to fix existing tests (not a big con really).
▪ Will have to decide what to do with new classes we introduce in the modules that have classes with old namespaces. Introduce with old namespace for consistency or with new namespace that matches module name to have correct namespace. Either way this might be a little confusing, and different from the case when we create brand new module with correct namespace
Beyond that, I think the only thing safe to assume is that the autoload section contains a configuration that will have composer generate an autoloader that will allow using the contained classes. Some packages for example contain more than one Magento module.