Someone with his early expansion locked down and unarguable?
Someone who marches pretty much everything out of his castle in the early game for maximum expansion figuring he doesnt need to worry until mid-game at earliest? Who figures he will see someone coming and can worry about defense then?
Why would such a person need to worry about Pangaea's invisible army?
The problem with it as an MP strategy would probably best be answered by some spreadsheet strategist. Im not that good with numbers so I cant quibble over the difference in costs of a few gold. I am sure that those folks can show you that the cost of creating and fielding an army that could take a castles basic defense AND be able to hold it against the returning home army MIGHT be possible but it would cripple you from actually being able to WIN the game so it is therefore a stupid strategy.