Well what he's saying is Riot implemented loot boxes as the free to play way to earn characters because loot boxes give highs and lows. Psychologically speaking, there's a tendency almost everyone has to get excited to open a box because it COULD give you exactly what you want. Conversely, not getting the champion you want puts you down mentally and that's when people are more likely to say screw it and skip the line.
With IP everyone knew it would take a while to get a character they wanted, but the grind was consistent. BE isn't consistent, can be high or low rolled, takes longer to get over time (every 25 levels), and overall is made to feel less rewarding because it's all luck. Worse, you can finally grind out a character then get a shard for them, making people more likely to just buy expensive characters next time.
This is all by design too. There's a reason loot boxes are in so many games now and it's because it psychologically manipulates people to buy more stuff. It's not even a question, that just is the case. It's no wonder new players aren't joining League, the couple friends I tried to get into it this year quickly got tired of the "loot box bullshit that's in every game" and quit. The game is already hard to learn and get people into due to the nature of its gameplay, and putting this system in up outside of the game people are trying to learn doesn't help.