Nexus I would suggest not paying attention to that youtuber's advice. What you're saying doesn't make sense. Some people choose not to use a SIM card in their phones because they don't want to be tracked by their cell service provider or because they have been misled about what SIM cards do.
Please see the project account's response to the OP question here:
GrapheneOS SIMs are extremely isolated and don't have control over the device. They're used to authenticate with a carrier. When the cellular radio and SIM are enabled, it authenticates your cellular service subscription with the cellular network. The cellular network inherently knows where you are based on the cell towers you're connected to.