The IsModuleInstalledOrRegistered task currently checks if the module node is present or if there's a module entry in the install context. However, module A can bootstrap module B node, thus fooling the task into believing that B is installed while it is actually not. Typical example for this is the observation module. Observation module is optional and no longer bundled by default and one often has ended up in a situation where some module bootstraps /module/observation/config/listenerConfigurations/foo thus implicitly creating a /module/observation node even if the module is not actually installed. Other modules subsequently using IsModuleInstalledOrRegistered(..) will thus act as if the module is installed even though it is not.
One way to make the check more reliable is to look for the presence of the version property under the module node. It's far from being foolproof but could be an improvement over the current situation.