It's similar to the stupid situation at Highbury and Islington where you cannot run a through service from the NLL to the ELL because you can't change voltage in a platform in case the wire breaks when the pan rises. The person that said that should have been confined to Farringdon station and made to wait until the wire breaks.
The person who said that was quite wrong, as far as I can work out.
The reason there are no through services is that there is no timetable capacity for trains heading east along the NLL to cross over onto the southbound ELL. The track layout along the NLL is that normal services uses the inner tracks, and the outer tracks are long freight loops.
Then simply because there are no through services planned they did not extend the OHLE into the platform at Farringdon much at all, it stops beyond the normal stopping point, ie not above the trains, and the route is not signalled for normal movements anyway.
It's chicken and egg. They haven't provided for traction changeover in the platform because it isn't needed, and someone's turned that on its head to say that it isn't there because it can't be done. Which is clearly not the same thing.
Changeover in the platform is routinely done at a number of other places in the LO system as you must be aware, and also at number of other places that aren't in platforms, whichever is best for the situation.