They have "get loud" type prompts all the time at Seahawks games. Volume 12 etc... Though, they often come at weird times, like during TV timeouts with the offense on the field, when fans are taking a break.
The guinness spokes holes said they would not be taking prompted recordings for the record though. They are apparently taking a few official readings when big plays happen.
In any case, KC will eventually win. They have more in attendance, and we will definitely lose a few decibels this year because of much higher second hand ticket prices. More money does not equal more rowdy. Don't believe me? Sit in club seats one game, then 300 level corner seats the next. Cheap seats is where the crazies live. Fewer cheap seats, means quieter fans, and more people telling you to sit down.
Prove me wrong 12th Man!
Edit: wow, weak. They apparently recorded the record during the commercial break after prompting the fans to yell. This has gotten cheaper and cheaper...