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.
Skill reasoning: Blind dart does not do much at level 1 for the pre-minion fights / very early game fights. For this reason Toxic Shot was chosen because the poison ticks over time and makes a player more likely to last hit minions (also does minor damage to champions which could be vital if a fight comes down to an enemy running with extremely low health). Move quick is optional but is very helpful at that low level because most players wouldn't have gone to get boots yet: allowing you to escape or catch up to auto attack once or twice more, adding more poison ticks.