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You did make it clearer the mistake people make though, the water flows out of the screw holes long before it goes over the top of the cistern. I assume the people who make the cisterns don't talk to the people that make the syphons, or they're made for a variety of cisterns. You can see why an installer would assume that a cistern and syphon supplied as a unit wouldn't overflow through the screw holes.
The other issue though is flow rate, if the overflow flow rate is lower than the incoming you have to lower the incoming rate. Happened to a friend on their top floor toilet while they were on holiday just after renovating the entire house, ruined every room in the house.
Airhead
Definitely do this, and make sure the internal overflow sits below the holes at the top of the cistern too. Whoever installed our bathroom failed to do either :|
e: read to the end, obviously you mention this because you're not an idiot. Unlike whoever installed our bathroom.