Using GrapheneOS without one AND aeroplane mode enabled (which turns off the cellular radio) is more secure since you are reducing remote attack surface from the cellular radio, SMS and others. SIM card is just used as authentication to that network and you are still taking part in it so you need both. Even if you have a SIM, using Aeroplane Mode but still using WiFi except when you need data is good practice (you're still connecting to your mobile network provider for WiFi calling and more). This also helps against cellular network tracking.

This has a huge usability cost for some users. High risk individuals are expected to disable radios (Bluetooth, UWB, WiFi, Cellular) when they aren't using them. It's an added measure from people with added caution.

Obviously messengers like Signal are out of the question with no phone number, so SimpleX is the first choice for this. 

 Thank you. A lot to learn here.