There isn't any evidence that they could do what Keynes was suggesting they could. I thought he was making the comment after the war, but I just looked at the date again and realised they were in the middle of WWII at the time! They most certainly did not have spare resources and couldn't possibly afford to be building houses with them. The example he's giving is a case where they couldn't afford it because they couldn't do it. If he'd picked an example where they could do it but people were claiming they couldn't afford it that'd be different and better. But if they could do something we'd probably also find that the financial figures suggesting they could in fact afford it.

"You can afford what you can do" is a truism - if you can do something, the financial system will tell you that you can afford it except if something really weird is going on. If the resources were there to build the houses then the prices would shuffle around until it was also affordable.