Everybody's reporting "Reddit had 430 million monthly active users in 2020", so that's the half-billion that seems to be fact-based.
Then the current 1.5B number seems to be based on extrapolation (1.66B in [1] for one estimate) based on previous growth rates.
I shared your initial skepticism, but Reddit is the 20th most popular site in the world [2]. I know I have definitely been surprised and even shocked that certain extended family members and coworkers of mine have turned out to be heavy Reddit users. It's turned into this incredibly widespread site that almost nobody talks about "in real life".
Of course, monthly active users presumably includes people who click on a Reddit search result once in the month. It doesn't mean they're using it daily and upvoting.
[1] https://www.bankmycell.com/blog/number-of-reddit-users/#sect...