They are similar in that they have the same form factor and compatible pinouts. This is a good thing because it means that accessories can work for a whole family of boards. For example, a "carrier" PCB designed for the classic Nano to plug in to can also be used with the Nano Every.
This sort of standardization has already proven to be hugely beneficial in that the form factor established by the original Arduino boards, which was used on the Uno, Leonardo, Uno WiFi Rev2, Mega, Due, Zero, and others has allowed the creation of a huge selection of shields and other accessories that can be mixed and matched with those boards.
So in the case of the Nano Every, the "Nano" is referring to the form factor. It is unfortunate that the classic Nano board and the Nano form factor share the same name, but I don't have any ideas about a better name for the form factor. I don't know know that the form factor of the Uno et al boards has ever been given an official name by Arduino, but I find myself calling it "the Uno form factor" anyway.