When talking about face decks being easy to play people aren't talking about the amount of work required to be a top tier face deck player, they're talking about the amount of work required to be a face deck player with a decent win rate. Imagine a normal distribution of player skill for each deck.

When first picking up a deck your average player is going to have a decent grasp of the general concepts of it simply through having the general skills associated with playing Hearthstone, and they're going to start off somewhere on the lower end of one standard deviation below the average player of that deck. As they play it and become more proficient in concepts specifically applicable to that deck they'll slowly chug their way up to being about a standard deviation above the average player of that deck in terms of skill.

For a low skill floor deck with a relatively flat skill plateau their journey will look something like this. They still have a long way to go if they want to claim to be a master of the deck, and they're a long, long way above those who play the deck poorly, but overall their skill journey hasn't significantly manifested itself as a large improvement in win rate.

Compare this to a deck with a higher skill floor and a less drastic plateau, looking more along the lines of this. What you end up with is a more identifiable space for improvement from your better than average player to a master of the deck, but more importantly than that, your average player's personal journey with the deck has netted a significant improvement in performance.

I can introduce someone to Hearthstone, teach them the fundamentals, then give them a short tutorial on face hunter and control warrior. The most probable result is passable performance with the face hunter deck, and them getting absolutely obliterated when making a large amount of sub-optimal plays with the control warrior deck. Not because they've mastered the face hunter and can climb to legend or go win tournaments with it, but because they've already passed the very low skill floor of understanding the types of plays needing to be made, with their number of misplays and how punishing they are being limited by the length of the game and the nature of the deck.

Meanwhile, and experienced face hunter player is going to win a face hunter mirror match against someone who mostly plays control warrior about as often as the control warrior will win the warrior mirror against the hunter player. Because both players have meaningfully improved skill with their main deck, even if both players are proficient enough with both decks to bring them to a tournament setting.

In short: It isn't about how much skill can go into playing a deck, but how meaningful the difference in performance is between the lower and higher end of the normally experienced skill range.