Actually, the article missed an importing part of the cleaning process.
The article stated" "...a bowl of water and some soap, get a good amount on the sensor and make sure you rub it in with your fingers really well"
But the mirror was not up in the video, so the instructor didn't actually scrub the sensor with his finger - just the mirror.
So how do you clean the sensor? Well, when submerged the battery may not work to flip the mirror. So the best way to clean the sensor is to get a long screwdriver - one that is 12" or more in length so it provides a lot of torque - then force it under the mirror then force the mirror up. You can hold the mirror up with the screwdriver while you scrub the sensor with your finger or a small brush with stiff bristles. If you use your finger and encounter a stubborn spot on the sensor, use your fingernail to chip it off.