Who made that figure up?

That's not anywhere near being true.

Club Wembley tickets are counted as sold and part of the announced attendance.

Well maybe free is taking it too far, or if away fans also get it free there needs to be some kind of redress from the away club. But there needs to be some kind of scheme to incentivise people to attend and since people seem to prefer a season ticket system (season ticket games are higher attended than non-season ticket) then targeting those people would seem a decent idea.
Maybe a discounted ticket plus bring a friend discount?

Why it's mostly £21 and £27 tickets they are empty blocks.

The reason for the long gap between the 1/4 and 1/2 is the BBC who have a mountain of sport to ram into those weeks and not helped this year with the commonwealth games. They could of course bung it on the red button imagine the uproar on here if they did that.

exactly!! If we want the competition broadcast live on BBC 1 we have to accept the scheduling that that affords the competition. With Wimbledon, the football World Cup and the Commonwealth games all taking place in July (when the natural spot for the cup semi would be) we were never going to be able to get a prime slot on BBC TV. So we either accept the timings available, or look to an alternative broadcaster.

Next year may be different, not sure what other sporting events they will have on BBC in July. We also will have to contend with the split for the Super 8's taking place at some point, and the probable need to slot the cup in before the split.

^^^this plus another couple of weeks and Leeds might have learned how to spell Castleford.