I think you are reading "beating" as higher user count. I read it as better features and user experience. But even in terms of user count, enough people are using third-party clients like Apollo that reddit deemed it worthy to kill them.
did some back of the napkin math with what the apollo developer has said publicly. each user makes 10,000 api calls a month and there are 2 billion calls per month which means 200,000 MAU. Reddit has an MAU around 500m users.
The article says the average Apollo user makes 345 API calls a month. Using your number of 2 billion calls per month, that’s more like 5.8 million MAU. Still around 1 percent of Reddit’s MAU.
EDIT: Oops, that’s per day! Your math is right, please ignore.