I suggest, given EDF area is down south, that this is another reaction to the London fire bridgades observation that they were responding to lots of consumer unit fires, that also lead to the move to metal CUs.
With modern cage clamp terminals, if the tails are terminated, and then wiggled, the cores will settle and the terminal will be loose again, getting on for 8-10 years ago, there was quite a lot of consumer unit fires, and it transpired that it was a combination of factors; due to the approval bodies trying to make proffit out of part P of the building regs and approving folk with basical quals and a quick assessment, a lot of inexpierienced installers were now changing consumer units, and not ensuring terminals were properly tight, and about the same time, manufacturers had been reducing fire retardants into the plastic of boards due to the REACH regulations, and then came the start of the smart meter roll out, and the meter fixers would have to move the tails about to fit the meter, potentailly loosing the connections if they were not tight to start with.
I imagine that in the case of a meter swap, they can probably come up with work arounds to fix the tails from moving too much, but not in the case of fitting an additional piece of equipment, movement has got to happen, so they perfer to play it safe and tell you you need to get someone else to reconnect tails
