But that's not the point. Regarding leveling up, I wouldn't have taken Air on level 4, but on level 5, when Earth was offered, I might have taken it.
I checked the entire skill tree, and if you don't take Air on level 4 and take Earth on level 5, it's a very good choice; there are no useless skills there.
But the Chinese player took Tactics on level 5, and my only explanation is that he didn't have many troops and wanted to take the monster lair with the maximum amount of units.
According to the guide, he ran to 121 for gold, a level-up, and to clear the path. It's not profitable to go back for the book there, and there's no money – buying units is more important.