It's not nonsense from me in terms of what the MK Technical Data Sheets have said for very many years (at least 10 years - see below). As I have said, although what they have written is potentially ambiguous (and MK Tech Support can't clarify that issue!), there are only two possible interpretations - that the total 'rating' of a double socket is either 26A or 13A (and, as EFLI and I have both said, the latter seems pretty unlikely).
As for your link, we have often discussed the quote from MK which appears within that thread. As you have shown, it appeared in the IET forum 10 years ago and, even then, no-one knew where the MK quote had come from - and people were saying (as you can see if you read that thread) that, even then, the stated 'rating' of MK sockets was, at that time, exactly as it is today (i.e. 13A "per socket outlet"), even though that material (presumably from MK at some point in history) said ...
All MK socket-outlets are manufactured to comply with BS1363 part 2: 1995 and are rated at 13A per unit. Double socket-outlets have been manufactured and tested to exceed this rating by margin that allows electrical safety and reduces the risk of heat and mechanical damage to components due to overloading. It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissible maximum loading and it should be remembered that double socket-outlets are not manufactured to be able to withstand a 26A load for sustained periods of time.
Research by ourselves and third party organisations has shown that all MK double sockets can safely withstand a continuous load of 19.5A for an indefinite period. Increasing the load slightly will begin to cause heat and mechanical stresses on the components in a relatively short period. Testing showed that a load of 22.3A was sufficient to cause heat stress that would cause a browning of the faceplates and sufficient heat to cause insulation damage to cable cores. A load of 24A for 43 hours was sufficient to cause significant heat damage to the material in which the socket-outlet was situated and within 75 hours sufficient to cause significant damage that would lead to the very real potential of fire.
MK recommend that users of their sockets consult professional design Engineers when designing installations to avoid the possibility of heat and mechanical stress to components and installations caused by overloading of MK socket-outlets.
Kind Regards, John