Yes it's just a myth .. the police are telling lies on their website to catch out drivers
Yes I'm aware the police don't have to follow those guidelines but they generally do.. which is why no one has EVER scanned and uploaded a NIP on motoring forums showing a NIP for going a couple of mph over the speed limit. I've seen posts where people have claimed they've been done for 2mph over the limit but when asked to show the NIP on the forum they don't ..EVER.
Also if it is just a myth I must be the luckiest man alive since I've been driving 25 years and gone through hundreds of speed cameras over the years at around 10% over the limit (I check I'm not above that when going through cameras) and never once revived a NIP. Before you say it, yes I know how much my speedometer over reads by (around 3mph) and I'm counting that too, so that means Ive often gone through 50mph speed cameras at 58mph shown on my speedometer

... just last month I went past a parked police speed camera van at 10% over too.
Cheers