Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
With this Garen build you will be able to whirlwind kids to their spawn zone. The 66 crit helps Whirlwind with mass damage with the added AoE damage of the Sunfire Cape. The frozen mallet will stop anyone caught in the whirlwind from running out allowing as much damage as possible. The Infinity's edge speaks for itself,and later in the game you can sell the avarice blades later in the game they were for the crit and extra gold to help you build your items quicker for an enemies demise.