I am wondering exactly the same. Maybe the standby costs? The chip needs to always have power to be able to receive the radio waves, it uses less power in that way but still quite a bit, I guess.

The surprise is how much power that is. It's either that or misleading marketing. I know Apple struggled to make this modem, so maybe it's still not great when it comes to standby power consumption. Since it's Apple we will only know after quite a bit of independent testing...

Phones maintain a cellular connection even if most data is being transmitted over WiFi at any given moment

Cellular modems are constantly reading and broadcasting messages to the cellular network unless they are explicitly turned off

But they use much more power when they are receiving data.

Sending data with a radio is always much more power intensive than receiving data unless the channel is extremely asymmetric in bandwidth and/or data rate.

Sure. But they also use a massive amount of power when they’re out of range so constantly searching for the network

Turning off your modem is second only to turning off your screen if you’re looking to save battery

Not nearly close enough to when they are receiving data, especially video.

When actively receiving data, a cellular modem will use significantly more power than when idle, with the exact amount depending on factors like network strength, data rate, and the specific modem technology, but typically ranging from several hundred milliwatts to a few watts