I would be very curious to know how one could test this so easily. https://www.imobie.com/icloud-unlock/locate-a-lost-cell-phon... and many articles demonstrate that even when a Google or IOS phone is off it can still transmit its location. This means it is not truly off and could possibly transmit more information than you realize. My custom OS phone does not do this TMK, and I am just SOL if I lose it. This is the price I am willing to pay.
An RF power meter or spectrum analyzer. It would be very clear if a device is radiating enough RF energy to make a connection.
Battery life is another giveaway, I've had android phones turned off in a drawer for months and they still have nearly 100% charge when turned back on. If they were using enough power to make any kind of cell connection, the battery would be dead in a few days.
The link you have there even says "When your phone is turned off or runs out of battery, you won’t be able to track its real-time location"
And this just posted to HN today to explain in detail for you how the Iphone stays on and transmitting when you think it is off. https://arxiv.org/abs/2205.06114v1
The FCC for one would throw a fit if airplane mode or power off still actively transmitted.