I'd tend to believe Riot on this one. My guess is it is seeing some decline, but not as precipitous as Dota 2.
Dota 2 has had the same problem since it officially released: a lack of ambition when it comes to new hero releases. When it was still in Beta, they were releasing heroes into the pool at a pace of a couple every 2 or 3 months. Boy did that change. New stuff is what keeps these games fresh for casual players, and casual players are the lifeblood of any multiplayer community game. League of Legends gets a new character every few months like clockwork, and they are usually marginally interesting. Dota 2 gets really unique and innovative characters, but for the last few years the game only gets those releases once a year at this point. MAYBE twice a year. That's just too slow a pace.
Further, Dota 2 has had a pattern over the last couple of years of introducing sweeping changes with huge change logs that have a major impact on how the game or its characters are played. Big map changes, big skill changes, new trees, and so on. This forces a major re-learning period that some people just don't want to engage in. Not after spending hundreds of hours just getting comfortable with how things were to start with. League tends to pump out a lot of small changes on a regular basis that are easier to digest and keep up with. It keeps players from feeling overwhelmed. The hat economy and trading changes also hurt the game.
That said, it is good that Valve is dropping more frequent patch updates, as there was a time when we'd go several months without any meaningful balance changes. A better release schedule and pace for new heroes would be wise, but I'm not sure if it can stem the trend of slowing player engagement unless they do something really interesting with the lore or some sort of event.