This is my interpretation too. After all, you can build a mine on the place where Iron WOULD be... but you'd see this really hard metal and say "damn, there's no way I can use this, it's too hard to shape it into anything!"
Maybe they hid all strategic resources before, but then they switched it up at the last minute? It's impossible to know. Balance is never 100% perfect. That's why there are patches and debates and so forth.
Potential Exploit
The problem with making any strategic resource visible right from the start is that the player can settle directly on the resource. This is smarter than building a worker, working the resource, and building a road to it. You may lose a turn at the beginning by moving your settler, but you save as much as 20 turns without requiring a worker. 20 turns can be a HUGE advantage for an early rush.
People were doing it with horses, imagine you could do it with copper. Chariots were showing up when other people had barely finished their first archer. Imagine that happened with axes. Not only was it impossible to counter, but it was ensuring that there were no other valid choices on the tech tree except to gun it for horses.
It's smart that they hid all strategic resources until a prerequisite technology. I don't know why they didn't do it sooner.