I have that feeling guy writing it was not an electrician and has cut down original to save space. Using mineral insulated cable one could use 1mm CSA and it is common to use MIC cable for industrial outside sockets. However these would not be part of a ring main and would not be fused at 32A. The BS7671:2008 gives a chart and that shows what is permitted and this shows 2.5mm cable for any socket no protected by a fused connection unit and 1.5mm when a fused connection unit is used.
The problem is always to identify if an extra socket can be added and if so what extra protection if any needs to be included.
I have tried many times to explain how to identify a ring main but I have found mistakes made even by electricians.
The disconnection times are very important but RCD and ELI testers are beyond the means of many a DIY installer often not even the polarity is tested.
However comments like "You should test the RCD before you use the socket each time by plugging in an appliance, switching it on, and pressing the test button." makes one think this is an old web page although the copyright says 2008 - 2010 but likely the page was written before that date and I think this is a problem with many web pages.
When I went to college to learn IT I wrote a few web pages as part of the course but now have no access to alter or delete these pages and the even on this forum one can go back to 1st Feb 2002 and read the posts. These are of course referring to a previous edition of BS7671:2008.
The site is British but they don't seem to have any proof readers they think Wrexham is a City for example.
They say
"An FCU is a socket with an integral fuse used for connecting high-wattage appliances, such as cookers" do they not know F is not proceeded with "An" it is "A" fuse connection unit. If they can't even correct grammar then how do you ever expect they get the technical bits right? Hope they have good insurance!
